Improve and simplify reusable block styles

[ZebulanStanphill]

Description

(Currently includes / blocked by #18902.) #18902 is now merged and has been removed from this PR.

Inspired by changes in #18105 and #18862 to reduce the UI weight of unselected blocks, I made this PR to simplify and improve the reusable block styles. I've made the following changes:

• Removed the reusable block indicator (<ReusableBlockIndicator />). Showing these indicators goes against the general idea of unselected blocks looking like the front-end, and they weren't very useful in the first place, in my opinion. You could hover the indicator to see the reusable block title, but this is a rather hidden aspect that I didn't even know about until making this PR, and it is easier to just select the block if you want to see its title. Additionally, it seems weird to keep the indicators now that the hover labels have been removed.
• Removed the dashed outline from unselected reusable blocks. This was another thing that seemed to add unnecessary visual weight. In my opinion there's no reason to distinguish reusable blocks from other blocks when unselected.
• Changed color of dashed outline for reusable blocks selected using the select tool. Previously, it was grey, but now it is blue to match how other blocks look when selected using the select tool.

I've considered changing reusable blocks to use solid outlines like every other block, but I'm not entirely sure about that change yet, so I'll leave it for another PR.

Screenshots

Note that when I took these screenshots, #18901 was not fixed in master, but it was in this branch. That's why the reusable blocks are taller in the "before" screenshots.

Unselected

Before:

After:

Selected (select tool)

Before:

After:

Selected (edit tool)

Before:

After (unchanged except for #18901 being fixed):

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• 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.

[jasmussen]

Very nice work, thank you! I left one question, but otherwise this seems good and tests well for me.

Reusable blocks as a whole provides an interesting design challenge worth exploring additionally, perhaps as part of efforts in #18667.

[ZebulanStanphill]

Addressed feedback.

[ZebulanStanphill]

I should note that there is currently a styling issue where the blue select color is not applied in dark theme mode, but this issue is also present in master and will probably require some more complicated specificity changes to fix, so I'll try dealing with them in a separate PR.

[ZebulanStanphill]

Rebased now that #18902 is merged. Will merge this PR after tests pass.

[ZebulanStanphill]

I created #18958 to fix the dark mode issue.