Bug: provided syntax object is not JSON-serializable, which causes ESLint to crash in certain situations

[nzakas]

This issue is being filed as a counterpart to humanwhocodes/tailwind-csstree#12.

The tailwind4 or tailwind3 syntaxes from the tailwind-csstree package are not JSON-serializable.

When ESLint tries to serialize the config (for example, when using the --cache or --print-config options), it crashes with the following error:

The requested operation requires ESLint to serialize configuration data,
but the configuration key "Value.theme" contains a function value,
which cannot be serialized.

Please double-check your configuration for errors.

Reproduction:
See: https://stackblitz.com/edit/vitejs-vite-7lu9csyw

Run npx eslint --cache or npx eslint --print-config style.css.

Reference:
Original issue: humanwhocodes/tailwind-csstree#12