Create literals and move all literals to literals.py file

[s3alfisc]

It would be nice if we consequently used string literals for type checks as in here.

To do so, we would set up a dedicated file, literals.py, and move all "stringly-typed" function arguments to the file.

We could start with the arguments to the feols and fepois APIs, for which we have the following stringly-typed arguments:

• vcov : Union[str, dict[str, str]] -> here "str" part could be either "iid", "hetero", "hc1", "hc2", or "hc3"
• weights_type: either "aweights" or "fweights"
• fixef_rm: "singleton" or "none"
• solver: "np.linalg.lstsq" or "np.linalg.solve"

cc @leostimpfle

Maybe of interest to @marcandre259 or @rafimikail ?

[marcandre259]

I've starting working on #557 with the weights for did, it's an issue that's also interesting for my work, so motivating to look at.

If @rafimikail or another contributor is actively working on #557 however, or once I get something done there, I would be happy to pick up this issue.

[s3alfisc]

Ah, very cool! I don't think Rafi is currently working on #557 so I don't think there are any double efforts at the moment =)