Styles: Convert px values to rem

[aristath]

Description

This was discussed in today's block-themes meeting and with the blessings of the team I'm starting a PR to slowly begin converting all styles to use relative units instead of absolute.

Please note: This is still a WIP.

Including @mapk as discussed in the meeting. This is a task that will probably require some coordination.

How has this been tested?

Hasn't been tested.

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.
• I've updated all React Native files affected by any refactorings/renamings in this PR.

[aristath]

Closing this one.
Instead of a huge PR it's preferable to do smaller ones and iterate as we go.
Smaller PRs have already been pushed and more are on the way.
(see #17076, #24322, #24323, #24330)