Refactor to add type annotations for s_expr.Expression

[aljazerzen]

In cases where we use field getter on schema objects, we have a custom mypy extension that figures out the type of the field. This does not work with Pylance unfortunately, so I went around and added type annotations.

- expr = pointer.get_expr(schema)
+ expr: Optional[s_expr.Expression] = pointer.get_expr(schema)

[msullivan]

I guess?

I'm not against adding any of the specific annotations, but I don't think we want to do this universally

[elprans]

What is the actual issue here? I disabled pylance complaints about this specifically in pyproject.toml. You should be running ms-python.mypy-type-checker in VSCode also to get proper typechecking for these (though not autocomplete sadly).

[aljazerzen]

I've used LSP "rename property" action, which didn't find all usages of the property.
I know that mypy works and I do have it in vscode, but there is no "go to definition", "rename property" or "find usages".