Distribute prebuilt VSIX via GitHub Releases

[dishmint]

Make installing the VS Code extension easier by shipping a prebuilt .vsix via GitHub Releases, so users don't have to build it from source.

Goal

Users can download a .vsix from the Releases page and install it with code --install-extension, without needing Rust or Node.js toolchains.

Scope

• GitHub Actions workflow that triggers on tag push (e.g. v*)
  • Builds jaw-lsp in release mode
  • Builds and packages the VS Code extension (npm run package)
  • Creates a GitHub Release and attaches the .vsix (and optionally the jaw-lsp binaries for major platforms)
• Update docs/installation/vscode.md to recommend the Releases download as the primary install path, keeping "build from source" as a fallback

Out of scope

• Publishing to the Microsoft Marketplace (separate future decision)

Part of #18.