[ty] IDE: add support for object.<CURSOR> completions (#18468)

This PR adds logic for detecting `Name Dot [Name]` token patterns,
finding the corresponding `ExprAttribute`, getting the type of the
object and returning the members available on that object.

Here's a video demonstrating this working:

https://github.com/user-attachments/assets/42ce78e8-5930-4211-a18a-fa2a0434d0eb

Ref astral-sh/ty#86

Author: BurntSushi