Add key= support for max() function

[Levtastic]

In python, you can add a function to a call to max() using the key= keyword arg, which lets you do more complex logic for finding the value you want, but in Transcrypt it looks like max() is mapped directly to Math.max() and so doesn't have this ability.

[Levtastic]

If this is added, I'd like to also request that it be added to functions like sorted()/list.sort() too, as I also use it in that situation a lot, as well as min()/max()

Edit: apologies, it seems that key is already supported for sorting!

[JennaSys]

Added in dev_3.9.3 branch to be released in v3.9.3

[JennaSys]

Merged to master on PR #876