[HTML5] Libraries refactor for linting.

Initial work to make liniting easier.

This includes:
- Rename http_request.js to library_godot_http_request.js.
- Rename externs.js to engine.externs.js.
- New library_godot_runtime.js (GodotRuntime) wraps around emscripten
  functions.
- Refactor of XMLHttpRequest handler in engine/preloader.js.
- Few fixes to bugs spotted by early stage linting.

platform/javascript/js/engine/engine.externs.js

@@ -0,0 +1,3 @@
+var Godot;
+var WebAssembly = {};
+WebAssembly.instantiate = function(buffer, imports) {};