Feature: Add font sizes values from core into theme json

[ian-pvd]

Description of the Change

This PR adds the font size values defined in core to the theme scaffold.
The core font sizes were retrieved from here: https://github.com/WordPress/gutenberg/blob/26556937b70bbf4aaef8c112321703b61a740284/lib/theme.json#L196-L222

Alternate Designs

My original goal was to override a single core font size while leaving the other values intact, but this isn't possible.

Benefits

• The font sizes that are available in Gutenberg by default will now be available to themes using the scaffold.
• Default font sizes supported by WordPress will now be listed in our theme.json, making it easier to customize them.
• Individual font size options can now be customized, without losing the other WP core font sizes.

Possible Drawbacks

• Detaches our theme's font sizes from the font sizes defined in WP core.

Verification Process

1. Installed a copy of the theme at trunk and confirmed that no font size options are availalble.
2. Made changes, rebuilt the theme, and confirmed that the font sizes I added were available.
3. Tried customizing a single font size option and was able to do so without losing or changing the other options.

Checklist:

• N/A I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

Applicable Issues

Relates to a conversation in Slack where I was trying to make changes to a single font size option, without losing the other WP core font size options.
https://10up.slack.com/archives/C8Z3WMN1K/p1638295806165800

Changelog Entry

Adds the default font-size options defined in core to the scaffold's theme.json file.