[ty] Add support for global __debug__ constant (#18540)

## Summary

Closes https://github.com/astral-sh/ty/issues/577. Make global
`__debug__` a `bool` constant.

## Test Plan

Mdtest `global-constants.md` was created to check if resolved type was
`bool`.

---------

Co-authored-by: David Peter <mail@david-peter.de>

crates/ty_python_semantic/resources/mdtest/scopes/global-constants.md

@@ -0,0 +1,14 @@
+# Global Constants
+
+## `__debug__` constant
+
+The [`__debug__` constant] should be globally available:
+
+```py
+reveal_type(__debug__)  # revealed: bool
+
+def foo():
+    reveal_type(__debug__)  # revealed: bool
+```
+
+[`__debug__` constant]: https://docs.python.org/3/library/constants.html#debug__

crates/ty_python_semantic/src/place.rs

@@ -1073,6 +1073,8 @@ mod implicit_globals {
             Place::bound(KnownClass::Str.to_instance(db)).into()
         } else if name == "__builtins__" {
             Place::bound(Type::any()).into()
+        } else if name == "__debug__" {
+            Place::bound(KnownClass::Bool.to_instance(db)).into()
         }
         // In general we wouldn't check to see whether a symbol exists on a class before doing the
         // `.member()` call on the instance type -- we'd just do the `.member`() call on the instance