Add rich-text source support for WordPress 6.5

[alecgeatches]

Description

See @leoloso's original PR in #57 (extended here) and the related Gutenberg PR: WordPress/gutenberg#43204.

The majority of html source type blocks have been converted to use rich-text. The main difference between html and rich-text seems to be that html supports multiline selectors, and rich-text has a different __unstablePreserveWhiteSpace supported setting.

The rich-text-specific __unstablePreserveWhiteSpace attribute is not currently implemented in the parser for a couple of reasons:

1. It's __unstable, which we don't currently use in any other parsing. If this attribute is standardized without the __unstable prefix, we should support this option.
2. This is only used in 3 blocks (<code>, <pre> and verse) to preserve copied newlines at present.

Steps to Test

This PR adds tests for rich-text attribute sources. Tests can be run via wp-env and composer:

wp-env start
composer install
composer run test