debian: Install emscripten

Here we add support for installing emscripten to the Debian images. Note that the user must source $EMSDK_ENV_SH to use the emscripten toolchain.

Addresses ghc#22128 (closed).

Note that we don't yet upgrade LLVM 15 as binaries are not yet available. Supposedly binaries will be uploaded soon.

components/Emscripten.dhall

+-- Install emscripten via the emsdk script. Sets EMSDK_ENV_SH to the path of
+-- the emsdk_env.sh script which can be sourced to use emscripten.
+
+let
+  Prelude = ../deps/Prelude.dhall
+let
+  CF = ../deps/Containerfile.dhall
+
+let 
+  install: CF.Type =
+      CF.run "install emscripten"
+      [ "git clone https://github.com/emscripten-core/emsdk.git"
+      , "cd emsdk"
+      , "git checkout 3.1.21"
+      , "./emsdk install latest"
+      , "./emsdk activate latest"
+      , "source ./emsdk_env.sh"
+      ]
+    # CF.env (toMap { EMSDK_ENV_SH = "/home/ghc/emsdk/emsdk_env.sh" })
+
+in
+  { install = install }
+