[system] Stabilizing CSS theme variable APIs

[siriwatknp]

Motivation

CSS theme variable feature was launched in v5.6.0 as an experiment. As time goes on, we gain more confidence that CSS variable is becoming a standard in web development, so this issue lists all the tasks to make the API stable in v6.

Todos

Some of the issues require further discussion:

• [system] Expose spacing unit as a CSS variable #37911 (fixed by [system] Add spacing theme token to be used in theme.spacing() #40224)
• [system] theme.getColorSchemeSelector is not a function #40178
• [material][CssVarsProvider] Fix HSL breaking button styles #39869
• Typography CSS vars are not generated by the CssVarsProvider #39677 [material-ui][joy-ui] Generate typography tokens #41703
• [material-ui][Paper] Need paperChannel along with defaultChannel when using extendTheme() #39740
  Add abillity to prevent CssVarsProvider from using local storage #35837

[siriwatknp]

I excluded #35837 from the initial release of v6. It's not a mandatory feature at this point and we can decide what it would be after the release.