Font Size Picker: Add default size

[mmtr]

Description

Adds a default option to the font size picker component that unsets any font size.

Previously, the font size selector was falling back to the normal size, but this size might have not been defined by theme.

Fixes #17908

How has this been tested?

• Activated the Twenty Twenty theme or any other theme providing font sizes not including a normal size (Twenty Twenty recently added a normal font size: Swap slug for regular font size to normal so it shows as default twentytwenty#989).
• Created a new post and added some paragraph blocks.
• Made sure the Text Settings on the right sidebar has the "Default" font size, with no font size applied to the block.
• Tried setting any other font size and made sure it worked as expected.
• Clicked on "Reset" and verified the selector changed to "Default", with no font size applied to the block.
• Removed the font size number (after toggling any font size) and verified the selector changed to "Default", with no font size applied to the block.

Types of changes

Bug fix

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.

[kwight]

Works great 👍

[jorgefilipecosta]

I left some comments, but the general idea looks good to me :)

[jorgefilipecosta]

Works well on my tests 👍