[ty] Make infer_subscript_expression_types a method on Type (#22731)

## Summary

A refactor in anticipation of
#22654.

Author: charliermarsh

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

@@ -432,3 +432,36 @@ def test4(val: Intersection[tuple[Foo], tuple[Bar]]):
     # TODO: should be `Foo & Bar`
     reveal_type(val[0])  # revealed: @Todo(Subscript expressions with intersections)
 ```
+
+## Intersection slice access
+
+```py
+from ty_extensions import Intersection
+
+class A: ...
+class B: ...
+class C: ...
+class D: ...
+
+def f(
+    x: tuple[A, B],
+    y: tuple[C, D],
+    z: Intersection[tuple[A, B], tuple[C, D]],
+):
+    reveal_type(x[1:])  # revealed: tuple[B]
+    reveal_type(y[1:])  # revealed: tuple[D]
+    # TODO: should be `tuple[B] & tuple[D]`
+    reveal_type(z[1:])  # revealed: @Todo(Subscript expressions with intersections)
+```
+
+```py
+class A: ...
+class B: ...
+class C: ...
+
+def g(x: tuple[A, B]):
+    reveal_type(x[1:])  # revealed: tuple[B]
+    if isinstance(x, C):
+        # TODO: should be `tuple[B]`
+        reveal_type(x[1:])  # revealed: @Todo(Subscript expressions with intersections)
+```

crates/ty_python_semantic/src/types.rs

@@ -122,6 +122,7 @@ mod visitor;
 mod definition;
 #[cfg(test)]
 mod property_tests;
+mod subscript;
 
 pub fn check_types(db: &dyn Db, file: File) -> Vec<Diagnostic> {
     let _span = tracing::trace_span!("check_types", ?file).entered();

crates/ty_python_semantic/src/types/generics.rs

@@ -705,27 +705,6 @@ impl<'db> GenericContext<'db> {
     }
 }
 
-#[derive(Debug, Copy, Clone, PartialEq, Eq)]
-pub(super) enum LegacyGenericBase {
-    Generic,
-    Protocol,
-}
-
-impl LegacyGenericBase {
-    const fn as_str(self) -> &'static str {
-        match self {
-            Self::Generic => "Generic",
-            Self::Protocol => "Protocol",
-        }
-    }
-}
-
-impl Display for LegacyGenericBase {
-    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        f.write_str(self.as_str())
-    }
-}
-
 /// An assignment of a specific type to each type variable in a generic scope.
 ///
 /// TODO: Handle nested specializations better, with actual parent links to the specialization of