remove async and await keywords

Also remove `@frameSize`, closing #3654.

While the other machinery might remain depending on #23446, it is
settled that there will not be `async`/ `await` keywords in the
language.

tools/doctest.zig

@@ -653,8 +653,6 @@ fn tokenizeAndPrint(arena: Allocator, out: anytype, raw_src: []const u8) !void {
             .keyword_align,
             .keyword_and,
             .keyword_asm,
-            .keyword_async,
-            .keyword_await,
             .keyword_break,
             .keyword_catch,
             .keyword_comptime,