Add type_replacements option to control output of name mangling

[YuhanLiin]

This PR introduces the mangle.type_replacements option, allowing users to replace type parameter names with their own strings when name mangling. This enables users to customize and control the output of the name mangling process.

[emilio]

Can you elaborate on the use cases for this that can't be done via just inserting a typedef or such in the prefix? In general I'm not thrilled to add another layer to mangling, this doesn't feel quite like mangling IMO.

[YuhanLiin]

It's mainly useful for generic enums, since it allows users to control how enum variant names are mangled. So something like enum Foo<Bar> { Variant } doesn't generate a name like FooVariant_Bar. In fact you can even disable the mangling altogether by replacing Bar with nothing, so you just get FooVariant instead. I think this is the only feature that can't be replicated with typedefs, so we could add it as a standalone feature instead.