Pattern Creator: Add theme.json to theme to enable appearance tools.

[ryelle]

This PR adds a theme.json file to enable appearanceTools, which enables all the possible UI controls for typography, colors, spacing, etc. It also enables the custom link color, since add_theme_support( 'experimental-link-color' ); doesn't seem to work.

Just adding a theme.json doesn't trigger wp_is_block_theme, so we don't need to worry about any conflicts or issues with the global header.

⚠️ There's a bug where the theme style overrides the link color when the container has a background, I'm not sure if this is a theme issue or not since I can't customize the link color on TT1 #55028-core.

Screenshots

New settings available in sidebar:

How to test the changes in this Pull Request:

1. Create a new pattern
2. See all the UI options available to you
3. Use them, they should appear on the frontend correctly

Note that this PR builds on #419 so that the styles are injected as expected.

[StevenDufresne]

Yep, it looks good. The one thing to note is that none of those inputs were showing until I enabled them. I would be surprised if many users are able to find them tucked behind the "..." vertical menu.

[ryelle]

The one thing to note is that none of those inputs were showing until I enabled them. I would be surprised if many users are able to find them tucked behind the "..." vertical menu.

Good point - that is how it works in gutenberg, so anyone used to working in GB should know to check the dots, but I'll see if I can get them to default on in the pattern creator (in another PR)