[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

crates/ty_server/src/server.rs

@@ -180,6 +180,7 @@ impl Server {
             completion_provider: experimental
                 .is_some_and(Experimental::is_completions_enabled)
                 .then_some(lsp_types::CompletionOptions {
+                    trigger_characters: Some(vec!['.'.to_string()]),
                     ..Default::default()
                 }),
             ..Default::default()