fix: respect language-specific dance.defaultMode overrides

[jariji]

Summary

• dance.defaultMode is declared as language-overridable in package.json, but observePreference reads it via getConfiguration("dance") without a document scope, so language-specific overrides like [magit] or [git-commit] are silently ignored
• The global value is stored as a single _defaultMode and applied to every new editor regardless of language
• Fix: resolve the default mode per-editor in _getDefaultModeForEditor(), passing the editor's document as the configuration scope so VS Code returns the correct language-specific value

Motivation

Users of edamagit (and similar extensions that register custom language IDs) cannot set a different default mode for those buffers. For example, this setting has no effect today:

"[magit]": {
  "dance.defaultMode": "insert"
}

After this fix, the above works as expected — magit views open in insert mode while other editors continue to use the global default.

Test plan

• Set "[magit]": { "dance.defaultMode": "insert" } in settings.json
• Open a magit status view via edamagit
• Verify the view opens in insert mode (not normal mode)
• Verify other editors still open in the globally configured default mode
• Verify that switching away from and back to a magit tab preserves whatever mode the user is in (fallback state is not affected)

🤖 Generated with Claude Code

[jariji]

I tested and confirmed this works locally.