Customize Direct Manipulation: Cleanup edit prompts and sync with core better

### What

In https://github.com/Automattic/customize-direct-manipulation/pull/53 we patched the code to enable the edit shortcuts for block widgets from core to take precedence. This was a quick fix, leading to code that feels a little prone to breakage (e.g. see [#L25-L30](https://github.com/Automattic/customize-direct-manipulation/blob/master/src/preview.js#L25-L30)) and that introduces a bit of styling contrast between the plugin's edit buttons and core's (see image below). The block widget prompts/buttons also transition a little differently than the header/nav/footer ones (not too profoundly though).

### How

Rough pointers/info:

- Potentially either introduce title/footer shortcuts in core (?) or use the new partials API in this plugin.
- Leave as-is for a while if there are plans to deprecate the edit prompts entirely in FSE (know nothing more beyond just mention of this in Slack: p1629457027242500/1629431515.236800-slack-CRWCHQGUB ). It's not _that_ severe style-wise.
- @yansern has done some work already on extracting just the guides in a [separate plugin](https://github.com/Automattic/customize-guide). So depending on which direction we go, we may use that.

Review PR #53 and src/preview.js, especially lines 25–30, to understand how the plugin currently yields to core’s block-widget edit shortcuts. Compare the plugin’s edit-button styling and transitions with core, then investigate the partials API and the separate customize-guide plugin. Done means an agreed direction that keeps prompts and core integration consistent without the current brittle behavior.

Written by the indexing model from the issue text.