Adds fluid typography minimum font size settings to wp_get_typography_font_size_value()

[andrewserong]

In ​https://github.com/WordPress/gutenberg/pull/42489 a feature was added to fluid typography in theme.json that allows a theme to specify a minimum font size to be used in fluid typography calculated font-size rules. The provided value ensures the fluid typography rules do not go below the settings.typography.fluid.minFontSize value. For example, if this is set to 16px then any font-sizes used at 16px or less will be output as-is without fluid typography rules being applied.

This feature is expected to land as part of WordPress 6.2, so it would be good to ensure that the PHP changes are backported in time for the release.

This PR backports the PHP changes in this Gutenberg PR: ​https://github.com/WordPress/gutenberg/pull/42489

The feature adds a settings.typography.fluid.minFontSize property to theme.json which allows themes to set a minimum font size for fluid typography calculations.

### Testing instructions

To test: edit the TT3 theme's theme.json file to swap out settings.typography.fluid from its current boolean value tor an object that specifies a minimum font size. For example, the following sets the minimum font size to 15px:

			"fluid": {
				"minFontSize": "15px"
			},

Then, in a post, create a few paragraphs and set a variety of custom font sizes. In the below screenshot of the markup, there are three paragraph blocks, set to 15px, 16px, and 20px respectively. Notice how the first block does not have fluid rules applied, and for the second and third blocks, the minimum size is 15px:

Note: this PR backports the PHP changes only. The update to support this feature within the block editor will be backported as part of the usual JS packages update. It should be pretty safe to land this PHP change first.

Trac ticket: https://core.trac.wordpress.org/ticket/57529

CI failures due to a missing closing ) on ​https://github.com/WordPress/wordpress-develop/pull/3866/commits/d56e8a88650348a1b477d299eedb4a78c85b24c7 🤦‍♀️

Confirmed the PR does backport the PHP changes from Gutenberg's /lib/block-supports/typography.php from ​Gutenberg's PR 42489 ✅

Note: The JavaScript changes will be handled when the packages are updated.

Confirmed the PR does backport the PHP changes from Gutenberg's /lib/block-supports/typography.php from ​Gutenberg's PR 42489 ✅

Note: The JavaScript changes will be handled when the packages are updated.

[hellofromTonya]

​As noted in the PR's approval, the PR does include all of the relevant PHP additions/changes from ​Gutenberg PR 42489. It includes integration tests.

Marking for commit. Self-assigning for the commit.

[hellofromTonya]

In 55133:

Editor: Adds 'settings.typography.fluid.minFontSize' support to wp_get_typography_font_size_value().

In wp_get_typography_font_size_value(), adds support for using a minimum font size for fluid typography when defined in a theme's theme.json file.

Reference:

• Part of ​Gutenberg PR 42489

Follow-up to [54497], [54260].

Props andrewserong, ramonopoly, hellofromTonya, joen.
Fixes #57529.

Committed via https://core.trac.wordpress.org/changeset/55133.

Wonderful, thank you for landing this one, Tonya! 🙇