Update toolchain keyword list to match design. (#2961)

- `xor` keyword is removed, with `^` used in its place.
- For consistency, also replaced unary `~` with unary `^`, as those
changes come from the same proposal.
- `type` keyword is added.
- To keep existing tests working, parsing support for `type` literal is
added too.
- `is` keyword is removed; we'd already added the `impls` keyword to the
list.
- Several other missing keywords added.

toolchain/parser/parser_handle_expression.cpp

@@ -70,7 +70,8 @@ auto ParserHandleExpressionInPostfix(ParserContext& context) -> void {
     case TokenKind::IntegerTypeLiteral:
     case TokenKind::UnsignedIntegerTypeLiteral:
     case TokenKind::FloatingPointTypeLiteral:
-    case TokenKind::StringTypeLiteral: {
+    case TokenKind::StringTypeLiteral:
+    case TokenKind::Type: {
       context.AddLeafNode(ParseNodeKind::Literal, context.Consume());
       context.PushState(state);
       break;