feat(styles): add support for a compat theme

[joshblack]

Closes #9176

Add in support for a compatibility theme in @carbon/styles. This should allow teams to use both v10 and v11 tokens together during migration.

The usage is outlined in styles/docs/sass.md, but the tl;dr is that a team can do the following:

@use '@carbon/styles/scss/compat/themes' as compat;
@use '@carbon/styles/scss/themes';
@use '@carbon/styles/scss/theme' with (
  $fallback: themes.$g100,
  $theme: compat.$g100,
);

body {
  // v11 token
  background: theme.$background;
  // v10 token
  color: theme.$text-01;
}

Changelog

New

• Add two new builders for compat themes and tokens entrypoints

Changed

• Update docs for styles to include the compat entrypoint

Removed

Testing / Reviewing

• Verify tests pass as expected
• Review docs and make sure they make sense and match usage in compat-test.js

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[tw15egan]

This looks great! 🎉 🔥

[kodiakhq]

This PR currently has a merge conflict. Please resolve this and then re-add the status: ready to merge 🎉 label.

[kodiakhq]

This PR currently has a merge conflict. Please resolve this and then re-add the status: ready to merge 🎉 label.