CodeQL model editor: Make "add" and "delete" buttons more intuitive

[shati-patel]

A minor UI suggestion for adding multiple models in the model editor. Previously, you'd click the last row of the model editor to add a new row, and then all other rows would become deletable, except that new empty one. We decided that it's slightly more helpful to have the first row remaining fixed (and undeletable).

Before:

After:

Checklist

• CHANGELOG.md has been updated to incorporate all user visible changes made by this pull request.
  • I don't think a changelog item is necessary here
• Issues have been created for any UI or other user-facing changes made by this pull request.
  • See internal linked issue
• [Maintainers only] If this pull request makes user-facing changes that require documentation changes, open a corresponding docs pull request in the github/codeql repo and add the ready-for-doc-review label there.
  • No update needed (I checked the docs + screenshot)

[shati-patel]

Some of the Windows tests are failing with

  ● Test suite failed to run

    spawn D:\a\vscode-codeql\vscode-codeql\extensions\ql-vscode\.vscode-test\vscode-win32-x64-archive-1.85.0\Code.exe ENOENT

I'm not quite sure why... It's similar to the errors I've been getting locally though (also on Windows) 🤷🏽

[robertbrignull]

Changes LGTM

I assume the windows test failures are random and not related to this PR. If they keep happening after retrying a few times we can look into whether this PR is special or look into fixing them generally.

[norascheuch]

I love this change!! I always found it super confusing that the last item wasn't deletable :D (Can you paste a screenshot of the new design to see a before - after?)

[shati-patel]

Tests finally passed after a few re-runs 🙃 🎉

(Can you paste a screenshot of the new design to see a before - after?)

Good idea! I've added a before/after to the PR description ❤️