Make examples runnable with jsaddle-warp

[georgefst]

Allows running examples locally without targeting WASM, e.g. with cabal run ghc-wasm-miso-examples -- snake. Note that it doesn't work on Firefox: ghcjs/jsaddle#64.

Code is largely adapted from Miso's own example. I don't know whether there's any particular reason this hadn't already been done in this repository. Maybe just that GHC's WASM backend is nowhere near as slow as GHCJS, so it's a bit less important? Anyway, I thought I may as well see whether you'd be interested in upstreaming this before going any further.

There are still quite a few things that could be improved:

• The two CPP branches in Main.hs are completely separate, so perhaps we're better off using two separate modules and Cabal conditionals instead.
• The CPP in App.hs is a bit awkward, and potentially overly complicated, since Data.JSString.Internal.Type.JSString is not the same as GHC.Internal.Wasm.Prim.Types.JSString. This is also the reason for the extra annotations in this file and XHR.hs: monomorphising the former gives us some compatiblity. It's possible that jsaddle could be improved to abstract over this difference.
• After a non-WASM build, building for WASM with ./build.sh requires a cabal clean. Presumably more could be done to keep build folders separate.
  • I'm not certain yet, but I think this issue has gone away since rebasing on main after Use TH-enabled build, remove custom miso #24.
• I've ditched the GHCI stuff (i.e. debug) from the Miso examples for now. This is simpler and easier to understand, since jsaddle-warp has a pretty bad and under-documented API, but it does limit some of the main benefits.
  • It works perfectly well with GHCI (EDIT: except that writing to stdout no longer works...), but when compiled the program immediately just exits.
  • I'm not sure what the purpose of Miso's debugOr 8080 (f >> syncPoint) jsaddleApp is. I've found that just using debug 8000 f works fine.
• CSS isn't picked up. We can host the CSS files using wai-app-static, replacing jsaddleApp with (jsaddleAppWithJsOr (jsaddleJs False) $ staticApp $ defaultWebAppSettings "frontend"), but I don't know how we can specify stylesheets like in our static HTML since we can't seem to get at the document's head.
• For some reason, fail "unknown example" isn't called when passing an invalid CLI argument.
• The XHR example is not yet implemented since it was not trivial to adapt.

[amesgen]

Code is largely adapted from Miso's own examples. I don't know whether there's any particular reason this hadn't already been done in this repository. Maybe just that GHC's WASM backend is nowhere near as slow as GHCJS, so it's a bit less important? Anyway, I thought I may as well see whether you'd be interested in upstreaming this before going any further.

Definitely interested, we even had jsaddle-warp support in this repo in the past, but removed it because it caused churn (see #6 (review)). Also, the fact that the WASM backend is much faster than GHCJS definitely is a part of the somewhat reduced benefit.

However, recent social media posts like https://redd.it/1g2lanx show that not everyone not aware of this possibility (which in particular gives HLS support), so I think it would be quite nice to demo this in this repo, thanks for (re)starting that effort! Will take a look soon.