Add multi-root workspace support

[mattmasson]

Summary

Enables VS Code multi-root workspace support. The main gap was to ensure that library symbol configurations in different workspace folders don't overwrite each other.

Changes

Server

• Enabled workspace folder support in LSP capabilities (workspaceFolders: { supported: true, changeNotifications: true })

Client — Per-folder symbol management

• LibrarySymbolManager: Replaced global registeredSymbolModules: string[] with per-folder Map<string, string[]>. Module names namespaced as folderKey::moduleName to prevent
  cross-folder collisions (e.g., two folders both having MyLib.json).
  • refreshSymbolDirectoriesForFolder(folderKey, directories) — only clears/reloads symbols for the specified folder
  • removeSymbolsForFolder(folderKey) — removes only that folder's symbols
  • removeAllSymbols() — clears all folders
  • refreshSymbolDirectories() — retained as convenience for single-folder/no-folder workspaces
• extension.ts: Reads additionalSymbolsDirectories config scoped to each workspace folder URI. Handles onDidChangeWorkspaceFolders to register/unregister symbols when folders are
  added or removed. Falls back to global config when no workspace folders exist.

Configuration

• Changed additionalSymbolsDirectories scope from machine-overridable to resource so different workspace folders can have different values

Tests

• 6 mock-based unit tests in librarySymbolManager.test.ts: per-folder namespacing, cross-folder isolation, folder removal, re-refresh behavior
• 6 multi-root integration tests in multiRootWorkspace.test.ts: runs in a separate VS Code instance with a .code-workspace file pointing to two fixture folders with different
  symbol libraries. Verifies extension activation, per-folder symbol loading, folder removal, and scoped config reading.
• Separate test run entry in .vscode-test.js for multi-root workspace tests (required — VS Code can only open one workspace type per instance)

Other

• Added .github/copilot-instructions.md (properly named) with build/test commands, architecture overview, and code conventions
• Removed old .github/.copilot.instructions.md (incorrect dotfile format)
• Ran npm audit fix across all packages

[mattmasson]

closing this in favor of #276