Switch to tree-sitter-bash for bash parsing

[CyberTailor]

The library currently uses a bash server that runs eval, which is a bit unnerving.

[CyberTailor]

The original idea comes from pkgcheck.

However, flit doesn't allow build steps, so another PEP517 backend should be used.

[CyberTailor]

Bash parser is currently used just for VDB environment files that don't require any evaluation and can be parsed statically.