`isforminstance` function for testing if something is 'instance' of a typeform

Ladies and Gentlemen, we got him.
```py
from typing import get_args, Literal

Letter = Literal["a", "b", "c"]
letters: tuple[Letters, ...] = get_args(Letters)

if "a" in letters: # good intent, but I don't wanna make a value for every type
...

if isinstance("a", Letter): # error
...

if isforminstance("a", Letter): # based
...
```

The issue names no files, tests, or entry points. Start by locating the repository's existing runtime typing components and determine the intended supported type forms for isforminstance; done means the proposed call works for the shown Literal example with documented behavior and coverage.

Written by the indexing model from the issue text.