Make await a template (#12085)

* Make await a template
* Generate await inside async/multisync

tests/async/tasync_noasync.nim

@@ -0,0 +1,15 @@
+discard """
+  errormsg: "undeclared identifier: 'await'"
+  cmd: "nim c $file"
+  file: "tasync_noasync.nim"
+"""
+import async
+
+proc a {.async.} =
+  discard
+
+await a()
+
+# if we overload a fallback handler to get
+# await only available within {.async.}
+# we would need `{.dirty.}` templates for await