Add reflex, jsaddle, ghcjs-dom, and related packages

This adds Reflex to Stackage as well as some packages that are
commonly used with Reflex. The goal is to make something like
“reflex-todomvc”[1] work out-of-the-box with Stackage. The ghcjs and
jsaddle dependencies are needed for running Reflex on a development
machine, but do not themselves require the ghcjs compiler.

These packages are fairly stable, and should be compatible with other
Stackage dependencies. I am not the maintainer for any of these
packages, but can work in keeping these up-to-date.

[1]: https://github.com/reflex-frp/reflex-todomvc

build-constraints.yaml

@@ -4042,6 +4042,17 @@ packages:
         - egison
         - mini-egison
 
+    "Matthew Bauer <matthew.bauer@obsidian.systems> @matthewbauer"
+        - ghcjs-dom
+        - ghcjs-dom-jsaddle
+        - jsaddle
+        - jsaddle-dom
+        - jsaddle-warp
+        - reflex
+        - reflex-dom
+        - reflex-dom-core
+        - reflex-vty
+
     "Grandfathered dependencies":
         - network
         - Boolean