Fix CSS theme() function resolution issue

[philipp-spiess]

We currently have three different implementations of the resolveThemeValue() Theme method:

1. The core version which only resolves based on the exact CSS variable name. This is the v4-only version.
2. A compat light version which attempts to translate a dot-notation keypath to a CSS variable name.
3. A full compat version which resolves the legacy theme object which is used whenever a @plugin or @config is added.

Unfortunately, there are some issues with this approach that we've encountered when testing the CSS theme() function while upgrading Tailwind Templates to v4 using our upgrading toolkit.

1. The compat light resolver was trying to convert theme(spacing.1) to tuple syntax. Tuples require a nested property access, though, and instead this should be convert to theme(--spacing-1).
2. We currently only load full compat version if @plugin or @config directives are used. It is possible, though, that we need the full compat mapping in other cases as well. One example we encountered is theme(fontWeight.semibold) which maps to a dictionary in the default theme that that we do not have an equivalent for in v4 variables.

To fix both issues, we decided to remove the compat light version of the theme() function in favor of adding this behavior to an upgrade codemod. Instead, the second layer now only decides wether it can use the core version or wether it needs to upgrade to the full compat version. Since typos would trigger a hard error, we do not think this has unintended performance consequences if someone wants to use the core version only (they would get an error regardless which they need to fix in order to continue).

[thecrypticace]

see comment about upgradeToFullPluginSupport