Implemented allowedUntypedLibraries config

[Wizzerinus]

Basically, this PR adds an allowedUntypedLibraries config field which is a list of strings (empty by default). If pyright cannot derive the type of a function, it normally gives an error (reportUnknownMemberType or reportUnknownVariableType), and if the library the method is implemented in is in this list, it doesn't, for example:

# both foo and bar are untyped, `allowedUntypedLibraries = ["first"]`
from first import foo
from second import bar
foo("123")   # ok
bar("456")   # error

Notes:

• Not documented right now
• maybe it's better to name this allowedUntypedModules instead of libraries
• maybe it would be good to allow configuring a list of modules deeper than 1 level (i.e. "library is strongly typed but library.new_submodule isn't"), although I think this is not necessary due to the uses of this feature (the point is that the rule normally targets the code written for the project, and this PR targets the code written for a library used in the project, which are usually not intersecting)

resolves #701

[DetachHead]

there are probably other things we will need to consider as well that haven't yet crossed my mind

[DetachHead]

nit: maybe move this comment to be directly above that new condition so it's more clear which part of the if statement it's referring to:

        if (
            !importResult.isStubFile &&
            importResult.importType === ImportType.ThirdParty &&
            !importResult.pyTypedInfo &&
            // If the module is allowed as an untyped library, we don't need the stub
            !this._ignoreUntypedModule(importResult.importName)
        ) {}

[DetachHead]

since this logic is repeated, perhaps it can be abstracted into a utility function located in ./packages/pyright-internal/src/analyzer/importStatementUtils.ts or something? (maybe thats not the best place for it so feel free to put it somewhere else if you can find a better spot)

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

AutoSplit (https://github.com/Toufool/AutoSplit): 198.02x slower (0.1s -> 18.3s in a single noisy sample)