fix(core): enforce JSON requests in FsModuleLoader (#36137)
## Summary
- reject non-JSON files when `FsModuleLoader` receives a JSON module
request
- return a `TypeError` with the actual module type and specifier
- cover JavaScript, extensionless, and valid JSON files
## Details
`FsModuleLoader` already rejected JSON files loaded without the JSON
import attribute, but it did not enforce the reverse mismatch. A `.js`
or extensionless file requested as JSON could therefore be returned as a
JavaScript module.
The loader now validates the requested and selected module types before
reading the source.
## Validation
- `./tools/format.js`
- `cargo check -p deno_core`
- `cargo test -p deno_core fs_module_loader -- --nocapture`
