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/htmx-experiment.cabal

@@ -0,0 +1,94 @@
+cabal-version:      2.4
+name:               htmx-experiment
+version:            0.1.0.0
+author:             Profpatsch
+maintainer:         mail@profpatsch.de
+
+common common-options
+  ghc-options:
+      -Wall
+      -Wno-type-defaults
+      -Wunused-packages
+      -Wredundant-constraints
+      -fwarn-missing-deriving-strategies
+
+  -- See https://downloads.haskell.org/ghc/latest/docs/users_guide/exts.html
+  -- for a description of all these extensions
+  default-extensions:
+      -- Infer Applicative instead of Monad where possible
+    ApplicativeDo
+
+    -- Allow literal strings to be Text
+    OverloadedStrings
+
+    -- Syntactic sugar improvements
+    LambdaCase
+    MultiWayIf
+
+    -- Makes the (deprecated) usage of * instead of Data.Kind.Type an error
+    NoStarIsType
+
+    -- Convenient and crucial to deal with ambiguous field names, commonly
+    -- known as RecordDotSyntax
+    OverloadedRecordDot
+
+    -- does not export record fields as functions, use OverloadedRecordDot to access instead
+    NoFieldSelectors
+
+    -- Record punning
+    RecordWildCards
+
+    -- Improved Deriving
+    DerivingStrategies
+    DerivingVia
+
+    -- Type-level strings
+    DataKinds
+
+    -- to enable the `type` keyword in import lists (ormolu uses this automatically)
+    ExplicitNamespaces
+
+  default-language: GHC2021
+
+library
+    import: common-options
+    exposed-modules:
+      HtmxExperiment,
+      Multipart,
+      ServerErrors,
+      ValidationParseT
+    hs-source-dirs: ./src
+
+    build-depends:
+        base >=4.15 && <5,
+        -- http-api-data
+        blaze-html,
+        blaze-markup,
+        bytestring,
+        conduit,
+        dlist,
+        http-types,
+        ihp-hsx,
+        monad-logger,
+        pa-error-tree,
+        pa-field-parser,
+        pa-label,
+        pa-prelude,
+        profunctors,
+        selective,
+        servant-multipart-api,
+        servant-multipart,
+        text,
+        unliftio,
+        wai-extra,
+        wai,
+        warp
+
+
+executable htmx-experiment
+    import: common-options
+    main-is: Main.hs
+
+    build-depends:
+        htmx-experiment,
+        base >=4.15 && <5,