Spike: Refactor style extraction mechanism

[danielmx-dev]

Description

The current style extraction mechanism has a couple drawbacks:

• It happens only once, at a very specific time during the site rendering. So it isn't reactive to changes. See this comment for an example: WooPayments card form fields are showing up in dark mode #10335 (comment)
• When not available through the config, the styles will be computed for every component using them, since they aren't centralized.

Acceptance criteria

• Define a solution that help us overcome the listed drawbacks.
• A proof of concept is desirable.

Dev notes

• Can we centralize the style extraction by defining a global store/Provider that will emit updates to react to changes?
• We could perform the style incrementally, so we can render the stripe form as soon as possible, then re-compute the styles once we make sure all CSS is fully loaded.
  • We can also react to other events, for example, in the site editor we can monitor if colors/background change and react accordingly.
• For classic checkout, we can call elements.update( { appearance } ); when styles change.