Heading block - add font family support

[Addison-Stavlo]

Adds __experimentalFontFamily support for the heading block.

For supporting fonts in FSE we need to consider which blocks most require the ability to have different fonts set than the main body. core/site-title and core/post-title both include support for font families.

Similarly, core/heading is a block that would also make sense for this. Generally, this support seems necessary for all blocks that serve the purpose of a title or heading (heading, site title, post title, etc.) to allow the same level of font style customization that was available in pre-FSE customizer themes.

Testing

• Run this PR
• Open the global styles panel -> blocks -> heading and verify we are able to select font family.

[aristath]

LGTM and makes sense... 👍 Having font-family controls in headings blocks is indeed something needed for designers

[mikachan]

LGTM. I think this is a great addition to the heading block.

[aristath]

3 approvals and no failing tests. I think this is good to merge 👍