Reintroduce styles=open parameter to open the styles panel directly

[luminuu]

What?

This PR reintroduces the styles=open URL parameter which, in combination with canvas=edit, enables to link to the styles panel directly from outside of the site editor. Fixes #49731.

Why?

This functionality existed before and has been removed when browse mode got introduced. I'm not sure if this was accidental or intended.

How?

Re-adding the code which has been added in this PR: #38093

Testing Instructions

1. Make sure to be logged in your WP admin
2. Add site-editor.php?canvas=edit&styles=open to the URL and hit enter
3. This URL should redirect you to the editing mode of the site editor with the styles panel being already open

Testing Instructions for Keyboard

See above.

Screenshots or screencast

[Mamaduka]

@ntsekouras, I believe you were working on something similar.

[ntsekouras]

@ntsekouras, I believe you were working on something similar.

Not exactly. My recent PR was to link to the block type panel when selecting a block with Styles open. I'd defer to @youknowriad for this one as I think he has the most context about the site editor browsing and whether this is deliberately absent.

Also I'd agree with @jasmussen's comment, that this could be considered resolved.

[youknowriad]

It's tricky indeed. Personally I feel that this is not needed because the "styles" panel is now in two places:

• The top level one which has a dedicated URL site-editor.php?path=%2Fwp_global_styles
• and the "inner" styles panel (the old global styles panel)

My opinion is that we should not bring this back as it's a very adhoc URL to open an internal panel. Why not an url to open the list view or the inserter... I think the browse mode being consistent and having a "styles" panel there too is a good alternative.