Upgrade: Migrate spacing scale

[philipp-spiess]

This PR adds migrations for the recent changes to the --spacing scale done in #12263.

There are a few steps that we do to ensure we have the best upgrade experience:

• If you are overwriting the spacing theme with custom values, we now check if the new values are multiplies of the default spacing scale. When they are, we can safely remove the overwrite.
• If you are extending the spacing theme, we will unset the default --spacing scale and only use the values you provided.
• Any theme() function calls are replaced with calc(var(--spacing) * multiplier) unless the values are extending the default scale.

One caveat here is for theme() key which can not be replaced with var() (e.g. in @media attribute positions). These will not be able to be replaced with calc() either so the following needs to stay unmigrated:

@media (max-width: theme(spacing.96)) {
  .foo {
    color: red;
  }
}

Test plan

We are mainly testing two scenarios: The JS config extends the spacing namespace and the JS config overwrites the spacing namespace. For both cases we have added an integration test each to ensure this works as expected. The test contains a mixture of keys (some of it matching the default multiples, some don't, some have different scales, and some use non-numeric identifiers). In addition to asserting on the created CSS @theme, we also ensure that theme() calls are properly replaced.