Site Editor: Validate the postType query argument

[noisysocks]

Description

Fixes https://github.com/WordPress/gutenberg/pull/36379/files#r753949616.

This bails if an invalid post type is provided via the postType query argument when loading the Site Editor's list view.

How has this been tested?

1. Open the site editor.
2. Click on the W menu.
3. Browse to Templates.
4. Change the postType argument in the URL to some gibberish e.g. asdf.
5. An error should appear.

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR (please manually search all *.native.js files for terms that need renaming or removal).

[kevin940726]

LGTM 👍

[talldan]

Do you think it's worth checking that it's one of the post types that the screen is intended for (wp_template or wp_template_part)?

[noisysocks]

Do you think it's worth checking that it's one of the post types that the screen is intended for (wp_template or wp_template_part)?

I think it's okay to not check this as edit.php doesn't do any validation beyond what I'm adding here. Really all that I'm concerned about is validating that there's nothing nasty in the query param e.g. HTML.

Plus, it's kind of neat? 😂 I could see us one day (ab)using this for reusable blocks or something.