Move the theme editor under tools for FSE themes

[youknowriad]

closes #36354

I hesitated between this or removing that page entirely (because the site editor is actually a different kind of theme editor) but I guess this is less disruptive.

Testing instructions

• Make sure the theme editor is under "tools" and not "appearance" for FSE themes.

[jameskoster]

It works:

We'll need to update that notice as well.

[carolinan]

I see that this change has the backport to WP Beta/RC label.
Please consider the comments in the existing Trac issue: https://core.trac.wordpress.org/ticket/54382, and whether that issue should be closed or not.

[youknowriad]

Oh I didn't know about that ticket, I'm happy to follow whatever approach is agreed upon. I think the decision about this is made.

[carolinan]

I think the decision about this is made.

Decisions need to be announced publicly, when they are not, discussions arise in multiple places and no one is able to keep up with both Trac, Slack and GitHub. The decision making needs to be transparent, even when we are all pressed for time.

[youknowriad]

Sorry, my message had a typo :). I meant "I thought" and not "I think", sorry for the confusion due to my broken English.