Support reading profiles from JsonSlabs files

[mstange]

Deploy preview (load this example jslb in it)

Fixes #6024.

JsonSlabs is an alternative serialization format that's more efficient than JSON if the serialized object contains typed arrays.
See https://github.com/mstange/json-slabs/ for details.

Our profiles currently do not yet contain typed arrays. But with this in place, we can start converting more and more tables / columns to use typed arrays, and incrementally reap the efficiency benefits.

This patch only adds the reading. In the UI, uploading / downloading profiles still uses JSON.

For the profiler-edit node script, this patch also adds the "writing": If you run profiler-edit with -o some-filename-ending-in.jslb, then it will create a JSLB file (or a compressed JSLB file for .jslb.gz).

---

The profile format "structure" is still the same as before. The format version is still given by profile.meta.preprocessedProfileVersion. Loading a JSLB file with a profile.meta.preprocessedProfileVersion that's
higher than the currently known one will still complain, and attempt to reload the page or the service worker.

This reload behavior is the primary reason why I want to get the "reading" part taken care of first, before we start converting any parts of the profile to get efficiency wins: The sooner we land this, the more likely it will be that, in the future, if somebody has a newer JSLB file but an old cached profiler, they'll see a useful error and
get the auto-reload.

Without this patch, loading a JSLB file in the profiler gives the following error:

Error: Unserializing the profile failed: Error: The profile array
buffer could not be parsed as a UTF-8 string.

---

With all that said, it turns out there is already a case where this format gives an efficiency win, thanks to its "split out" feature that lets us put certain subtrees of the profile into separate JSON slabs: If the original JSON is larger 512MiB, we can split it into individual slabs which are each smaller than 512MiB, so the built-in JSON.parse will work and we don't have to use the slow streaming parse.

Example:
https://storage.googleapis.com/profiler-get-symbols-fixtures/large-speedometer3-profile.json.gz
expands to a 577.74MiB JSON file,
https://storage.googleapis.com/profiler-get-symbols-fixtures/large-speedometer3-profile.jslb.gz
expands to a 564.66MiB JSLB file (slightly smaller because compacting removed unused strings)

The JSLB file breaks down as follows:

idx  type          bytes  elements  path               
---  -------  ----------  --------  -------------------
  0  json      16.76 MiB         -  .                    (root)
  1  json     264.82 MiB         -  .shared.stackTable 
  2  json      91.01 MiB         -  .shared.frameTable 
  3  json       8.47 MiB         -  .shared.funcTable  
  4  json      28.80 MiB         -  .shared.stringArray
  5  json     154.80 MiB         -  .threads           
---  -------  ----------  --------  -------------------
     6 slabs  564.66 MiB

All of those individual JSON slabs are now under 512MiB.

Loading the JSON file, uses streaming parser: https://share.firefox.dev/3PcB9vK (19 seconds)
Loading the JSLB file, uses native JSON.parse: https://share.firefox.dev/3RkFcXs (2.3 seconds, 8.3x faster)

---

The example jslb next to the deploy preview link was generated as follows: yarn build-node-tools && node ./node-tools-dist/profiler-edit.js --from-hash 29apty4565r8dec0hrjfty68w88rexc6na6bn1r -o ~/Downloads/normal-profile.jslb.gz

---

Follow-ups:

• Land Rust tools in the json-slabs repo to list .jslb contents / convert to JSON / expand to directory (I have most of this written already)
• Allow uploading jslb files to profile storage
• Add UI to the sharing panel to allow downloading as either json or jslb
• Actually start migrating tables / columns to typed arrays
• Change uploading to use jslb instead of json
• Update samply / fxprof-processed-profile to support outputting to jslb

[codecov]

Codecov Report

❌ Patch coverage is 81.81818% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.80%. Comparing base (41473d3) to head (206f8ff).
⚠️ Report is 12 commits behind head on main.

Files with missing lines	Patch %	Lines
src/node-tools/profiler-edit.ts	71.42%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6037      +/-   ##
==========================================
+ Coverage   83.77%   83.80%   +0.02%     
==========================================
  Files         329      329              
  Lines       34423    34567     +144     
  Branches     9627     9570      -57     
==========================================
+ Hits        28839    28969     +130     
- Misses       5155     5169      +14     
  Partials      429      429              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.