Fix typing of LetterCase

[mtnpke]

Multiple issues prevented the letter_case argument on the dataclass_json decorator to work correctly with strict type checking settings and PyRight:

• While config() etc. correctly accept also a callable instead of only an enum member, the dataclass_json decorator did not do this. Since the enum members are mapped to callables, which cannot be used as base types, they cannot be used as argument values of type LetterCase.
• All functions in stringcase.py were missing type hints.
• The LetterCase enum members need concrete type definitions, so that there arguments are sufficiently constrained and they are compatible with the correct callable type Callable[[str], str]. PyRight would otherwise infer Callable[..., str], which is not narrow enough.

These changes allow a JSON dataclass with letter_case argument to pass type testing with basedpyright. The code is fully backwards-compatible, as only type information is enhanced. Since dataclasses-json includes the py.typed marker file, it would be helpful if the type info was as complete and useful as it can be.

Test case:

from dataclasses import dataclass
from dataclasses_json import dataclass_json, LetterCase

@dataclass_json(letter_case=LetterCase.CAMEL)
@dataclass
class Test:
    name: str

Result before:

$ uvx basedpyright pyrighttest.py
c:\_GIT\3rdparty\dataclasses-json\pyrighttest.py
  c:\_GIT\3rdparty\dataclasses-json\pyrighttest.py:4:29 - warning: Type of "CAMEL" is partially unknown
    Type of "CAMEL" is "(string: Unknown) -> (Unknown | Literal[''])" (reportUnknownMemberType)
  c:\_GIT\3rdparty\dataclasses-json\pyrighttest.py:4:29 - error: Argument of type "(string: Unknown) -> (Unknown | Literal[''])" cannot be assigned to parameter "letter_case" of type "LetterCase | None" in function "dataclass_json"
    Type "(string: Unknown) -> (Unknown | Literal[''])" is not assignable to type "LetterCase | None"
      "function" is not assignable to "LetterCase"
      "function" is not assignable to "None" (reportArgumentType)
1 error, 1 warning, 0 notes

Result after:

$ uvx basedpyright pyrighttest.py
0 errors, 0 warnings, 0 notes