gh-117705: Add follow_symlinks to os.path.exists()

[nineteendo]

Benchmark:

::test.bat
@echo off
echo existent && python -m timeit -s "import ntpath" "ntpath.lexists('test.bat')" && PCbuild\amd64\python.exe -m timeit -s "import ntpath" "ntpath.lexists('test.bat')"
echo non-existent && python -m timeit -s "import ntpath" "ntpath.lexists('foo.bar')" && PCbuild\amd64\python.exe -m timeit -s "import ntpath" "ntpath.lexists('foo.bar')"

existent 
5000 loops, best of 5: 47.9 usec per loop # before
5000 loops, best of 5: 46.8 usec per loop # after
# -> 1.02x faster
non-existent
10000 loops, best of 5: 21.8 usec per loop # before
20000 loops, best of 5: 16 usec per loop # after
# -> 1.36x faster

• Issue: Add follow_symlinks to os.path.exists() #117705

---

📚 Documentation preview 📚: https://cpython-previews--117729.org.readthedocs.build/

[nineteendo]

@serhiy-storchaka, thoughts?

[serhiy-storchaka]

How does it differ from lexists()?

[nineteendo]

It's more explicit, and it allows us to implement a faster lexists() on Windows without duplicating C code.