Add web workflow to build Repl with Emscripten

[zeux]

Fixes #177

The build artifact produced after this change can be attached to releases; https://luau-lang.org/demo references a specific pinned release (0.505 at the moment) to fetch the JS binary.

We can probably change the JS code later to try to find the latest release using a GitHub API or binary search...

[dcope-rbx]

lgtm 😊

[tiffany352]

We can probably change the JS code later to try to find the latest release using a GitHub API or binary search...

There's a relatively simple way to get info about the latest release in json format:

curl -H 'Accept: application/json' https://api.github.com/repos/Roblox/luau/releases/latest

{
  // snip
  "tag_name": "0.505",
  "target_commitish": "master",
  "name": "0.505",
  "draft": false,
  "prerelease": false,
  "created_at": "2021-11-19T16:10:07Z",
  "published_at": "2021-11-19T16:14:25Z",
  // snip
  assets: [
    // snip
    {
      "url": "https://api.github.com/repos/Roblox/luau/releases/assets/50013249",
      "id": 50013249,
      "node_id": "RA_kwDOCN7VCM4C-yRB",
      "name": "Luau.Web.js",
      // snip
    }
  ]
}

[zeux]

@tiffany352 Nice! Is this something the browser can just query, or does this run into rate limiting et al?
We could probably also use this as a default and fall back to the hardcoded version.

PRs welcome in either case ;-)