Comments embedded in an instruction's keyword that spans multiple lines confuses the parser

[rcjsuen]

The instruction should be merged into one instead of being split off because of the comment. This regression was introduced by the fix for #98.

FR\
#
OM

HE\
#
AL\
#
THCHECK

import { DockerfileParser } from "dockerfile-ast";
let dockerfile = DockerfileParser.parse("FR\\\n#\nOM");
console.log(dockerfile.getInstructions()[0].getKeyword());
dockerfile = DockerfileParser.parse("HE\\\n#\nAL\\\n#\nTHCHECK");
console.log(dockerfile.getInstructions()[0].getKeyword());

FR#
HE#