Support Literal[True] and Literal[False] types

[Siyet]

Reopening #996 (auto-closed when fork was deleted).

Summary

• Add support for Literal[True], Literal[False], and Literal[True, False] in type definitions
• Introduce MS_TYPE_BOOLLITERAL_TRUE / MS_TYPE_BOOLLITERAL_FALSE flags to distinguish bool literals from plain bool
• Handle PyBool_Type before PyLong_Type in typenode_collect_literal (since bool is a subclass of int)
• Update decode paths (JSON, msgpack, convert) to validate against specific bool literal values
• Update inspect.py LiteralType.values type annotation to include bool

Closes #859

[ofek]

Nice, thanks!

[Siyet]

Two follow-ups pushed:

• Docs (docs/supported-types.rst) - added bool to the list of supported Literal value types, plus a couple of examples.

• Error messages (src/msgspec/_core.c) - while testing the doc examples I noticed validation errors for Literal[True] / Literal[False] produced messages with an empty type name (Expected '', got bool). Fixed all three decode paths (mpack_decode_bool, json_decode_true/false, convert_bool) to emit Invalid enum value <val>, consistent with how int/str literals are reported. Test added in test_literal_bool_error_message.

Side note: the Invalid enum value wording is misleading for Literal types in general since they are not enums. I opened #1009 to track that and submitted #1010 (based on this branch) to fix it across all Literal kinds at once.

[Siyet]

The failing build job here is unrelated to this PR: it's the link checker tripping on the Pydantic docs redirect (docs.pydantic.dev/latest/ → pydantic.dev/docs/validation/...), which is fixed in #1008. Once #1008 lands and this branch is rebased, CI should go green.