Refactor nu-check

[kubouch]

Description

This PR refactors nu-check and makes it possible to check module directories. Also removes the requirement for files to end with .nu: It was too limiting for module directories and there are executable scripts around that do not end with .nu, it's a common practice for scripts to omit it.

Other changes are:

• Removed the --all flag and heuristic parse because these are irrelevant now when module syntax is a subset of script syntax (i.e., every module can be parsed as script).
• Reduced code duplication and in general tidied up the code
• Replaced unspanned errors with spanned ones.

User-Facing Changes

• nu-check doesn't require files to end with .nu
• can check module directories
• Removed --all flag

Tests + Formatting

After Submitting

[fdncred]

Thanks for cleaning this up. People have been asking for this. IIRC, there are a few issues this will close.