feat: compile .vyi interfaces as types

[iphelix]

What I did

Added support for compiling standalone .vyi interface files into ABI-only contract types so they can be used by ape scripts. This is similar to how JSON abi interfaces are handled.

fixes: #134

How I did it

• Removed previous filter for interface files for the Vyper 0.4 output selection.
• Set custom output selection of ["abi"] for interface files
• Run a separate vyper -f abi on interface files

NOTE: Keeping files under interfaces/ import only as before and not exposed to ape scripts.

How to verify it

Added regression coverage:

uv run pytest tests/functional/test_compiler.py -k "standalone_vyi"

Checklist

• All changes are completed
• Change is covered in tests
• Documentation is complete