Move code to handle Tuple and TypedDict `__getitem__` to plugins

[Michael0x2a]

Currently, we handle calls to both Tuple and TypedDict's __getitem__ code inside checkexpr. We should consider turning some or all of this code into a plugin.

I think it's somewhat debatable how useful this refactor would be for Tuple's __getitem__, but it would at least make the way we handle TypedDicts more consistent: we currently handle all other TypedDict methods inside of a plugin.

Also see #6124 (comment) for context.

[ilevkivskyi]

Note that it is better to fix #6117 first to avoid making it even more problematic.