Support type hints and `py.typed`

[bkeryan]

Is your feature request related to a problem? Please describe.
Running mypy on a project that uses nisyscfg returns errors like this:

error: Skipping analyzing "nisyscfg": module is installed, but missing library stubs or py.typed marker  [import-untyped]

Describe the solution you'd like
Add type hints for the parts of nisyscfg that don't have them, and add a py.typed file so that clients using mypy or Pyright can benefit from nisyscfg's type hints.

Describe alternatives you've considered
None

Additional context
None

Workaround

[[tool.mypy.overrides]]
module = ["nisyscfg.*"]
ignore_missing_imports = true