javascript: Add is [not] nullish for loose null check

[auscompgeek]

These check whether a LHS is nullish, i.e. null or undefined.

This is intended to complement #1255.

[AndreasArvidsson]

I wonder if we just want to update these actions instead?

community/lang/javascript/javascript.py

Line 76 in 8aeed40

def code_insert_is_not_null():

The number of times you want to check just for null, but not undefined is low so I think that "is null" probably should do != null by default.

[auscompgeek]

I do agree it's pretty rare for me to check for specifically null or undefined. With #1255 it'll likely be confusing though.