gobbleIdentifier() {
		let ch = this.code, start = this.index;

		if (Jsep.isIdentifierStart(ch)) {
			this.index++;
		}
		else {
			this.throwError('Unexpected ' + this.char);
		}

		while (this.index < this.expr.length) {
			ch = this.code;

			if (Jsep.isIdentifierPart(ch)) {
				this.index++;
			}
			else {
				break;
			}
		}
		return {
			type: Jsep.IDENTIFIER,
			name: this.expr.slice(start, this.index),
		};
	}
Gobbles only identifiers
e.g.: `foo`, `_value`, `$x1`
Also, this function checks if that identifier is a literal:
(e.g. `true`, `false`, `null`) or `this`
@returns {jsep.Identifier}