Compat-transpiler: implement `u` flag

[DmitrySoshnikov]

The u flag should already be implemented by most of the engines, however, there are still old engines which do not implement it, so it'll still be nice to have it in compat-transpiler (ES2015 -> ES5).

There is an explicit implementation for the u flag in regexpu package. Potentially can be ported to regexp-tree.

[DmitrySoshnikov]

@mathiasbynens, you should know the topic, and details well, as an implementer of the regexpu, will be glad to get your reviews on this (in case you'd be willing to port it, I think you'd be the best owner for this feature).

[mathiasbynens]

I’ve been meaning to port regexpu-core to a regexp-tree plugin and run some comparisons! Consider this assigned to me.

[DmitrySoshnikov]

Sounds great, thanks! Let me know if you need any help, or any feature (and of course feel free to add any in case).