Support ** exponentiation operator from ECMAScript 2016.

Available in both expressions (a 2) as well as
assignments (a = 2).

Lifted from work done at... work.

Also applied some 0 -> nullptr changes there were forgotten in the
original .y file when patching the grammar.cpp file.

grammar.cpp got generated via the following command:

/opt/bison-2.7/bin/bison -o grammar.cpp --defines=grammar.h -pkjsyy grammar.y