exempt data URL CSS from Pages Router global CSS restriction (#89901)
### What?
Skip the Pages Router global CSS restriction for `data:` URL imports
(similar to app router and mixed app/pages router)
### Why?
`data:text/css` should work the same no matter if pages router, app
router or mixed router usage
### How?
the validation in `crates/next-api/src/module_graph.rs` already had a
carve-out for `node_modules`. This adds the same for data URL modules.
fixes #89900
- [x] Related issues linked using `fixes #89900`
- [x] Tests added
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Co-authored-by: Tim Neutkens <tim@timneutkens.nl>