Support name mangling

[zhihaoy]

Add the functionality to perform name mangling from an AST.

Introduced three types of nodes to assist this purpose better:

1. Node('abominable'): Handle function type with cv and ref qualifiers, distinguishing that from a cv/ref-qualified type. The associated change also fixes a bug when formatting methods.
2. CastNode('entity'): Still calling it a cast since it spells L...E. Passing a function as a non-type & can be supported by this without introducing the full-expression.
3. Node('oper_unary'): Need that to mangle unary +, -, &, * differently.

Unsupported features:

1. Template argument substitution (need to access _Cursor, or analysis identity of Python objects)
2. Template parameter pack (need to distinguish that in the AST)
3. Special entities (the patch focuses on exported APIs)

Unverified:

1. Exported variables (may be a low-hanging fruit)
2. Certain Clang-specific extensions

Future improvements:

1. Allow users to provide the standard library names in the AST without __1 but still get name mangling that suits libc++ using some knob.

The change also moved the project to use pyproject.toml, locked pytest in a test group using uv, and migrated the tests to pytest.