[ty] Infer more precise types for collection literals (#20360)

## Summary

Part of https://github.com/astral-sh/ty/issues/168. Infer more precise types for collection literals (currently, only `list` and `set`). For example,

```py
x = [1, 2, 3] # revealed: list[Unknown | int]
y: list[int] = [1, 2, 3] # revealed: list[int]
```

This could easily be extended to `dict` literals, but I am intentionally limiting scope for now.

crates/ty_python_semantic/resources/mdtest/subscript/lists.md

@@ -9,13 +9,11 @@ A list can be indexed into with:
 
 ```py
 x = [1, 2, 3]
-reveal_type(x)  # revealed: list[@Todo(list literal element type)]
+reveal_type(x)  # revealed: list[Unknown | int]
 
-# TODO reveal int
-reveal_type(x[0])  # revealed: @Todo(list literal element type)
+reveal_type(x[0])  # revealed: Unknown | int
 
-# TODO reveal list[int]
-reveal_type(x[0:1])  # revealed: list[@Todo(list literal element type)]
+reveal_type(x[0:1])  # revealed: list[Unknown | int]
 
 # error: [invalid-argument-type]
 reveal_type(x["a"])  # revealed: Unknown