Don't enable execCommand commands in EditContext

[dandclark]

Closes https://github.com/w3c/edit-context/issues/71.

Update execCommand spec per resolution of https://github.com/w3c/edit-context/issues/71#issuecomment-1806317341 to indicate that execCommand commands are not enabled in EditContext, except for commands that are enabled regardless of whether the command is performed in an editable region.

The following tasks have been completed:

• [X] Confirmed there are no ReSpec/BikeShed errors or warnings.
• [X] Modified Web platform tests: https://github.com/web-platform-tests/wpt/commit/5411d9cadd5b8dcdd7c84c053f8420b105fc2a90

Implementation commitment:

• [ ] WebKit (haven't yet implemented EditContext)
• [X] Chromium: https://chromium-review.googlesource.com/c/chromium/src/+/4975435
• [ ] Gecko (haven't yet implemented EditContext)

[snianu]

Could we add some WPT tests for this?

nvm. I just realized that you added tests in third_party/blink/web_tests/external/wpt/editing/edit-context/edit-context-execCommand.tentative.https.html