Create python bindings for in process checks

[tylerriccio33]

Summary

This draft PR is to pair #3964 and show an example of how the in process API could work, with bindings directly to the rust code via pyo3. I can't think of another way to do this without adding a new crate and pyo3, which could be undesirable from the dependency standpoint.

Perhaps this functionality should be made optional?

Fixes #3964

Test Plan

I add a simple rust test and a python test to go with the crate. I wanted the python test to capture the fact we have the pyrefly.toml file and the helper file which we import from. Another nice thing (which @yangdanny97 pointed out) is this plays off lsp mode so any future checks would be against the warm checker

If this direction is approved I'll of course drastically expand the test suite

def test_checker_resolves_imports_and_reports_errors(tmp_path):
    (tmp_path / "pyrefly.toml").write_text("")
    (tmp_path / "helper.py").write_text("def greet() -> int: ...\n")

    checker = pyrefly_api.Checker(project_root=str(tmp_path))

    diags = checker.check("from helper import greet\nx: str = greet()")
    assert [d.kind for d in diags] == ["bad-assignment"]

    assert checker.check("from helper import greet\ny: int = greet()") == []

FYI - I used claude to help me figure out how the playground worked, since I was having trouble determining a few things. Honestly never worked in codebase this size in rust

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