feat(plugins): commit workflow API for plugins

Plugin API 1.3 adds what commit-message generators and before-commit checks need:

- `menu = "commit.message"`: a plugin command, with the plugin's icon, in the toolbar above the commit message of the Commit tool window and the Commit dialog.
- `commit.pending-commit()` (`vcs.read`): what Commit would commit right now, per file its repository, path, status and a `git diff` of only the included files and changed blocks against HEAD (1 MiB in all), plus the message, Amend and a revision. The diff is made by the same selection the commit uses, in a copy of the index, so the index and the working tree stay as they are; unsaved editors of the included files are saved first, as Commit does. Only in trusted projects.
- `commit.set-commit-message(revision, message)` (`vcs.write`): replaces the message only while the draft is still at that revision, so text the user typed meanwhile is never overwritten (`stale`).
- `[[contributes.commitChecks]]` and the optional `commit-checker` export (worlds `plugin-commit-checks` / `commit-checks`): checks listed in Commit Options (on by default, `vcs.commit.disabledPluginChecks` turns them off) and run after the IDE's own; their problems, optionally with a location, stop the commit until Commit Anyway.

The window answers the host through `CommitRequested` / `plugin_answer_commit`, also for remote windows. The Rust SDK gains `host::pending_commit`, `host::set_commit_message`, `openidea_plugin::commit` and their mock; a `commit` test fixture exercises them. All components are rebuilt for the WIT change.

Refs #1650

wit/plugin.wit

@@ -17,6 +17,7 @@ world plugin {
     import editor;
     import invocation;
     import vcs;
+    import commit;
     import events;
     import tasks;
     import diagnostics;
@@ -73,6 +74,12 @@ world plugin-services {
     export service-provider;
 }
 
+/// A plugin with before-commit checks (`[[contributes.commitChecks]]`).
+world plugin-commit-checks {
+    include plugin;
+    export commit-checker;
+}
+
 /// Only the `syntax-provider` export: guests generate it next to `plugin`
 /// (the component tools merge both into one component).
 world syntax {
@@ -106,3 +113,8 @@ world hover {
 world services {
     export service-provider;
 }
+
+/// Only the `commit-checker` export, generated next to `plugin` like `syntax`.
+world commit-checks {
+    export commit-checker;
+}