Rename Type -> type, per #2360. (#2507)

Also make minor updates to the skeletal design in
docs/design/name_lookup.md following #2113, as there are no longer any prelude names that are made available to unqualified name lookup by default.

Add `type` to the keyword list in
docs/design/lexical_conventions/words.md, following #2360.

explorer/testdata/array/element_convertible_to_type.carbon

@@ -10,8 +10,8 @@
 package ExplorerTest api;
 
 class TypeLike {
-  impl as ImplicitAs(Type) {
-    fn Convert[self: Self]() -> Type { return i32; }
+  impl as ImplicitAs(type) {
+    fn Convert[self: Self]() -> type { return i32; }
   }
   fn Make() -> Self { return {}; }
 }