Do not leak crate-private type SourceFingerprint

[dahlbaek]

Compiling on rustc 1.71.1 currently triggers a compile-time error:

error[E0446]: crate-private type `SourceFingerprint` in public interface
   --> compiler-core/src/language_server/router.rs:190:5
    |
190 |     pub gleam_toml_fingerprint: SourceFingerprint,
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ can't leak crate-private type
    |
   ::: compiler-core/src/build.rs:415:1
    |
415 | pub(crate) struct SourceFingerprint(u64);
    | ----------------------------------- `SourceFingerprint` declared as crate-private

For more information about this error, try `rustc --explain E0446`.

This pr implements the staightforward fix, marking gleam_toml_fingerprint crate-private.

[dahlbaek]

closes #3162

[lpil]

Could you make the whole class crate private please