FontSizePicker: Allow custom units

[aaronrobertshaw]

What?

Updates the FontSizePicker to allow the customization of the available units when displaying the custom font size control.

Why?

It might benefit theme authors to be able to control the available custom font size units.

How?

• Adds support for a units prop that can be used to set the available units for the custom font size control.
• Updates the README to match latest preferred format and add missing size prop

Next Steps

This PR came about through a discussion on filtering or disabling units within the custom font size picker for the editor's typography panel. If we add support for custom units to the FontSizePicker component we'll still need to update our typography panel to handle passing the desired units to the component.

Testing Instructions

1. Run npm run test:unit packages/components/src/font-size-picker and ensure everything still passes
2. Load Storybook (npm run storybook:dev) and check the FontSizePicker examples function
3. Within the controls section of the With Units example update the preset font sizes to match the allowed units
4. Update the units prop to the desired units e.g. [ 'em' ]
5. Switch the control to the custom control view and confirm the units available match the units prop.

Note: If you do not update the font sizes you might have a value set in the control containing a non-allowed unit such as px which would show in the available units until the value is changed or control is reset.

Screenshots or screencast

Screen.Recording.2023-02-27.at.1.50.59.pm.mp4

[glendaviesnz]

Worked as advertised for me

[github-actions]

Flaky tests detected in 079056f.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/4348986647
📝 Reported issues:

• [Flaky Test] should navigate block hierarchy using only the keyboard #47640 in specs/editor/various/block-hierarchy-navigation.test.js