Migrate UI to a PolyGerrit repo-command web component

The plugin UI still used the GWT-era screen API: the static
JavaScript called self.onAction('project', 'parent', ...) with the
popup action context. PolyGerrit removed onAction() long ago, so the
plugin loaded but never showed any UI.

Replace it with a Lit web component in TypeScript, built with the
same toolchain as the delete-project plugin: a web/ directory with a
ts_project compiled against the shared plugins tsconfig base, rolled
up by gerrit_js_bundle into static/reparent-project.js and attached
to the plugin jar as a resource jar. Because the bundle keeps the
old file name, HttpModule and its JavaScriptPlugin binding are
unchanged.

The component registers at the repo-command endpoint. It renders
only when the server-side UiAction is present in config.actions,
which preserves the permission-based visibility of the old popup:
the action is exposed only to users passing the plugin's
canReparent() check. The dialog offers the new parent project and an
optional commit message, and issues the same
PUT /projects/{name}/reparent~parent request as before.

Change-Id: I7d94e2c8f1b60a5372840df9ce1b7a46e8d05c31