Better JS debugging experience (2)

Following #9035, there's still a few things to take care of I think:

- temp var name generation : I'm not sure if the patch addressed that, the idea is to reduce the amount of _g1,_g2 etc generated in output code. Adding variable name mapping in sourcemaps would be even better since we could either remove them entirely or collapse them into a single "" name.

- stepping : when stepping with F10, we often stay on the same Haxe line and have to step several times to go to the next line. I'm not sure if that's a problem with sourcemaps or if we need to change JS output so one line of Haxe gets generated within the same JS line.