feat: integrate skill into URL handler

[bencoz]

Description

This PR adds support for automating the installation of "skills" (multi-file folders) via the chat-skill/install URL handler. This enables a streamlined workflow where users can install complex skills by clicking a single link (e.g., vscode:chat-skill/install?url=...) that points to a JSON manifest.
This PR resolves #290735

The handler:

1. Fetches and parses the JSON manifest.
2. Uses the name property from the manifest to determine the local folder name (ensuring consistency and validity).
3. Recursively creates the directory structure.
4. Downloads all files specified in the manifest.

Changes

• PromptUrlHandler.ts:
  • Added handling for the chat-skill/install path.
  • Implemented logic to fetch and parse the JSON manifest.
  • Added validation for the skill name (must exist in manifest, must be a valid filename, must not already exist on disk).
  • Implemented a loop to download multiple files and create necessary subdirectories (mkdir -p behavior).
  • Added a specific security confirmation dialog for skill installation.

Validation

valid-layers-check: Pass
compilation: Pass

Testing

1. Host a JSON manifest (e.g., as a GitHub Gist) with the following structure:

   {
     "name": "algorithmic-art",
     "files": [
       { "source": "https://raw.githubusercontent.com/.../SKILL.md", "destination": "SKILL.md" },
       { "source": "https://raw.githubusercontent.com/.../script.js", "destination": "templates/script.js" }
     ]
   }

2. Trigger the installation via URL:
   vscode:chat-skill/install?url=<RAW_URL_TO_MANIFEST>
3. Select a destination skill folder when prompted.
4. Verify that the folder algorithmic-art is created and contains the files with the correct structure.

Example manifest file:

https://gist.githubusercontent.com/bencoz/b3e03b778c3892127e36e0fe3da2a0d6/raw/80be47aa6d9e86c6acde993ccfab1e932cac0bca/algorithmic-art-skill.json