[ty] support accessing __builtins__ global (#18118)

## Summary

The PR adds an explicit check for `"__builtins__"` during name lookup,
similar to how `"__file__"` is implemented. The inferred type is
`Any`.

closes https://github.com/astral-sh/ty/issues/393

## Test Plan

Added a markdown test for `__builtins__`.

---------

Co-authored-by: David Peter <sharkdp@users.noreply.github.com>

crates/ty_python_semantic/resources/mdtest/scopes/moduletype_attrs.md

@@ -15,6 +15,15 @@ reveal_type(__package__)  # revealed: str | None
 reveal_type(__doc__)  # revealed: str | None
 reveal_type(__spec__)  # revealed: ModuleSpec | None
 reveal_type(__path__)  # revealed: MutableSequence[str]
+reveal_type(__builtins__)  # revealed: Any
+
+import sys
+
+reveal_type(sys.__builtins__)  # revealed: Any
+
+from builtins import __builtins__ as __bi__
+
+reveal_type(__bi__)  # revealed: Any
 
 class X:
     reveal_type(__name__)  # revealed: str