Editor mode config option

[zth]

This sets up an -editor-mode flag for bsc. The intention is that the editor tooling can set this as it calls bsc for the incremental compilation that powers the continuous feedback without saving in the editor.

It's mostly a placeholder for now, but it does set some configuration that's good for the editor tooling already:

• ignore_parse_errors lets the compiler continue even if it finds parse errors.
• cmi_only stops after type checking, so it never runs the JS generation related optimizations and the generation itself. This is because the generated JS is not interesting to the editor tooling, only the type checking (and type artifacts) are. Should in theory speed things up a little since bsc will do less work.

But mostly, this is a mode that we can use as we do more stuff in the compiler intended for the editor tooling.