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/function/auto_return/modify_arg_type.carbon

@@ -9,7 +9,7 @@
 
 package ExplorerTest api;
 
-fn Id(t: Type) -> auto { return t; }
+fn Id(t: type) -> auto { return t; }
 
 // Test non-trivial type expression in parameter type.
 

explorer/testdata/function/auto_return/modify_return_type.carbon

@@ -9,7 +9,7 @@
 
 package ExplorerTest api;
 
-fn Id(t: Type) -> auto { return t; }
+fn Id(t: type) -> auto { return t; }
 
 // Test non-trivial type expression in return type.
 

explorer/testdata/function/auto_return/type.carbon

@@ -9,7 +9,7 @@
 
 package ExplorerTest api;
 
-fn Id(t: Type) -> auto { return t; }
+fn Id(t: type) -> auto { return t; }
 
 // Test non-trivial type expression in variable declaration statement.
 

explorer/testdata/function/fail_parameter_type.carbon

@@ -9,7 +9,7 @@
 package ExplorerTest api;
 
 // 42 cannot be used as the type of a parameter.
-// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/function/fail_parameter_type.carbon:[[@LINE+1]]: type error in type of name binding: 'i32' is not implicitly convertible to 'Type'
+// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/function/fail_parameter_type.carbon:[[@LINE+1]]: type error in type of name binding: 'i32' is not implicitly convertible to 'type'
 fn f(x: 42) -> i32 {
   return x - 1;
 }

explorer/testdata/function/fail_recurse_before_typecheck.carbon

@@ -8,7 +8,7 @@
 
 package ExplorerTest api;
 
-fn T() -> Type {
+fn T() -> type {
   // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/function/fail_recurse_before_typecheck.carbon:[[@LINE+1]]: attempt to call function `T` that has not been fully type-checked
   var v: T() = 0;
   return i32;

explorer/testdata/function/fail_return_call_has_invalid_body.carbon

@@ -8,7 +8,7 @@
 
 package EmptyIdentifier impl;
 
-fn apply[T:! Type, U:! Type](f: T, EmptyIdentifier: U) {
+fn apply[T:! type, U:! type](f: T, EmptyIdentifier: U) {
   // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/function/fail_return_call_has_invalid_body.carbon:[[@LINE+1]]: only arrays and tuples can be indexed, found bool
   match (true[true]) {}
 }

explorer/testdata/function/fail_var_type_is_call.carbon

@@ -13,7 +13,7 @@ fn test() -> i32 {
 }
 
 fn Main() -> i32 {
-  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/function/fail_var_type_is_call.carbon:[[@LINE+1]]: type error in type of name binding: 'i32' is not implicitly convertible to 'Type'
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/function/fail_var_type_is_call.carbon:[[@LINE+1]]: type error in type of name binding: 'i32' is not implicitly convertible to 'type'
   var x: test() = 1;
   return 0;
 }

explorer/testdata/function/return_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 {}; }
 }