Add GMX to runner targets

[adam-coster]

Now that we're more likely to target GMX for upcoming projects like "How Many Dudes?", it's a good time to revisit all the runner stuff.

Still sticking with things that can be compiled on Windows:

• GMX (web target using WASM)
• HTML5 (As requested Allow the Runner to use the HTML5 export platform #274). Low priority since HTML5 is a janky export target and GMX accomplishes the same goal but in a much less janky way.

[adam-coster]

@BscotchSeth Should I put some gas on adding GMX as a build target in Stitch?

[BscotchSeth]

@adam-coster Definitely ignore the HTML5 stuff. It's so deprecated and un-maintained that it's not worth the hassle.

Adding a GX.Games build target in Stitch would be quite good, since we could then iterate on web builds in Stitch instead of GameMaker. But we still have a few weeks probably before we'll be doing that, so it can go a bit farther down your list.