Check whether editor commands are declared in package.json

[colin-grant-work]

What it does

Fixes #11763 by checking whether commands registered via registerTextEditorCommand are declared in package.json and registering only a handler, if so.

How to test

1. Install and activate CLangD.
2. Observe that no warnings about redundant registration of clangd.typeHierarchy or clangd.ast are logged.
3. Observe that the clangd.ast command still works as expected: right clicking in a C(++) editor should show a Show Ast menu item, and clicking it should add a view container part to the Navigator that shows the AST at that place in the file.

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[msujew]

The changes look good to me and I can confirm that the warnings no longer appear 👍