Closes#17346
This PR fixes an issue that caused the `--theme(…)` function to behave
differently after a legacy JS plugin or config was configured. The issue
was that the compatibility layer would patch the theme value resolver to
always inline the value. This, however, is only expected to happen if
the path does not look like a CSS variable in which case this legacy
code path should not be run.
To fix this, I'm now keeping a reference to the regular theme resolution
function and call into it if the path starts with `--`.
## Test plan
- Tested with the repro in #17346 by adding pnpm overrides and confirmed
that this fixes the issue
- Added a unit test to the `--theme(…)` resolution tests
eec1bf2b84