feat(editors/vscode): add VS Code extension with syntax highlighting, snippets, and CI

[Morbden]

Summary

• Adds the initial VS Code extension for Galfus Script (editors/vscode), implementing syntax
  highlighting via TextMate grammar, code snippets, language configuration, and packaging/validation
  scripts.
• Needed to give developers first-class editor support when working with .gfs/.gfp files.

Key changes:

• Grammar overhaul (galfus.tmLanguage.json): reworked all rules to use proper TextMate
  scopes (storage.type, entity.name.function, entity.name.type, variable.language, etc.);
  added declarations group that highlights fn/struct/enum/choice/constraint/type/var/const
  together with the declared name; added backtick string, spread operator (...), arrow (=>),
  null-coalescing (??) and improved number regex (supports underscores and scientific notation);
  fixed pattern priority order so operators no longer shadow keywords.
• Language configuration (language-configuration.json): fixed lineComment from invalid
  object form to plain string "//" (was silently rejected by VS Code); added backtick as an
  auto-closing/surrounding pair.
• Snippets (snippets/galfus.json): 10+ snippets covering fn, struct, enum, choice,
  constraint, type, if/else, for, match, typeof, and transaction.
• Scripts (scripts/check.ts, scripts/pack.ts): validation script that loads the grammar
  and asserts scope tokens against fixture files; packaging script that wraps vsce.
• CI (.github/workflows/editors-ci.yml): workflow that runs grammar validation on every push
  and PR touching editors/vscode/**.
• Assets: brand and icon PNGs added to assets/ and copied into editors/vscode/assets/ for
  the extension marketplace icon.
• .bun-version pinned at the repo root for consistent tooling.

Linked Issues

• Closes None

Change Type

• changelog:breaking
• changelog:feature
• changelog:fix
• changelog:refactor
• changelog:performance
• changelog:docs
• changelog:internal (exclude from public changelog)

User-facing Changelog Note

• Added VS Code extension for Galfus Script with syntax highlighting, code snippets, and automated grammar validation CI.

Risks / Rollback

• Main risks: grammar regex regressions on edge-case syntax not yet covered by fixtures; marketplace publishing credentials not yet configured.
• Rollback plan: revert the branch; the extension is not yet published so no user impact.