Simplify layer structure hierarchy plumbing #3742
Description
See fn serialize_structure and fn serialize_root:
They implement an over-engineered encoding of the hierarchical depths of a list of layers.
And see function newUpdateDocumentLayerStructure:
It implements the decoding of that over-engineered encoding into a JS format.
We want to replace this complexity with a much simpler flat structure of structs that become JS objects using the same way this happens with all our other data types sent from Rust to JS (via serde and messages.ts).
Instead of encoding things "cleverly" using a custom schema documented in that verbose comment in the first code snippet link, where negative signs correspond to hierarchy level shifting and such-and-such, it should just use the simplest form that directly describes the same concept but without the bit fiddling and custom schema.