Generate ES6 arrows without this temp vars

[RblSb]

Removes this temp vars for ES6 mode, functions that need function-scope this can be marked with

callback(@:js.function function() { // generate `function` in js instead of `() => {}`
// uses js.Lib.nativeThis, js.Syntax.code, etc
});

This is also port of pf_can_capture_this branch, so this capturing state is:
js-es5/python not supported
js-es6/php works
eval/lua/jvm/hl/cpp requires additional fixes, currently disabled

Closes #10352
See #9773

[kLabz]

I'm really not sure we'd want to do that 🤔
Also seeing js-specific (new) code in typer doesn't feel right