Add support to compile to JavaScript with emscripten

[eahumada]

Request to add support to the atari800 emulator to be compiled to JavaScript using the emscripten technique that is used by jsA8E and JSMESS.

Currently there is not a JavaScript emulator for Atari XL that be practical to use to produce content and integrate in pages.

Use atari800 as JavaScript emulator could be great to the future creation of web based Atari content and development tools.

[eahumada]

As an additional comment: Having a JavaScript or WebAssembly target will be great to be able to generate live demos or online tools like the shown here: https://8bitworkshop.com/v3.7.1/?platform=atari8-800xl.mame&file=hellopm.dasm

[mikrosk]

Patches welcome. ;)

[JeremyJStarcher]

I once submitted a patch that would allow this as part of an SDL-2 conversion.

That patch has now gone stale, however, and I am looking at manually merging it into the latest version from here.

[mikrosk]

@JeremyJStarcher feel free to create a PR. Closing as resolved.