feat(users/Profpatsch): init HtmxExperiment

I’m playing around with htmx (server-side html snippet rendering),
this is a simple registration form and some form validation that
happens in-place.

Change-Id: I29602a7881e66c3e4d1cc0ba8027f98e0bd3461c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8660
Tested-by: BuildkiteCI
Reviewed-by: Profpatsch <mail@profpatsch.de>
Autosubmit: Profpatsch <mail@profpatsch.de>

users/Profpatsch/htmx-experiment/Main.hs

@@ -0,0 +1,4 @@
+import HtmxExperiment qualified
+
+main :: IO ()
+main = HtmxExperiment.main