Add template part previews to placeholder block

[Addison-Stavlo]

Description

An alternative exploration to #22612 as a potential step forward for issue #21218. Here, I have added the previews and selectable interface to the Template Part placeholder block. A handful of styles and use-async-list file were carried over from the inserter implementation.

The placeholder now has 2 sections divided by Tabs: one for selecting via previews and the other for creating the new template part.

The template part creation flow has been 'mostly' untouched for the time being. For this section I have removed the preview, updated the choose/create button and function to only support 'create', and updated the help text to guide the user.

This seems like a good step forward, but this will probably require some future iteration in style and creation flow in upcoming PRs.

How has this been tested?

Local docker environment in site/post editors.

Types of changes

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.

[epiqueras]

Let's follow this up with: #22760 (comment).

[diegohaz]

I'm seeing this error when running npm run build:

Error: Undefined variable: "$theme-color".
    at options.error (/Users/haz/Projects/gutenberg/node_modules/node-sass/lib/index.js:291:26) {
  type: 'Error',
  '$error': '$error',
  status: 1,
  file: '/Users/haz/Projects/gutenberg/packages/block-library/src/template-part/editor.scss',
  line: 67,
  column: 32,
  formatted: 'Error: Undefined variable: "$theme-color".\n' +
    '        on line 67 of packages/block-library/src/template-part/editor.scss\n' +
    '        from line 46 of packages/block-library/src/editor.scss\n' +
    '>> \t\t\tborder: $border-width solid $theme-color;\n' +
    '\n' +
    '   -------------------------------^\n'
}

In fact, it seems like $theme-color is not defined. Maybe a rebase issue?

Update: #23217

[noahtallen]

Maybe a rebase issue?

it must have been something in the merge since the build was passing here

[epiqueras]

Super weird.

[Addison-Stavlo]

I am seeing this in travis builds on another PR (#23007), but I started seeing it a couple hours before this PR merged? its definitely confusing me here...

[Addison-Stavlo]

I cant build master because of this error. I wonder why $theme-color works in various places in the inserter:

gutenberg/packages/block-editor/src/components/inserter/style.scss

Line 138 in 3797d17

color: $theme-color;

but now having it here is failing.

[noahtallen]

maybe a file needs imported?

[Addison-Stavlo]

#23221 should fix it.

And that permalink I noted above was on the wrong branch, it looks like they changed to var(--wp-admin-theme-color) 11 hours ago.