Adding a custom html file for WASM build

[CashWasabi]

When compiling for emscripten target there is an option to also use a custom html file. I think this could be enabled by adding an argument to the linkWithEmscripten function inside emcc.zig. I already tested it out with a harcoded path and it works.

Is this something you would like to add?

If you want I could just create a PR that then enables the option to parse a custom html file if needed.

The benefit here is that you can e.g. just have a simple canvas instead of the template html file which is normally generated.