function goNext( c ) {
	if ( c && c !== ch ) {
		error( 'Expected `' + c + '` instead of `' + ch + '`' );
	}
	ch = text.charAt( at );
	at += 1;
	return ch;
}
Returns the next character and assigns it to the `ch` variable. When there are no more characters, the function returns the empty string.

## Notes

-   If a `c` parameter is provided, the function verifies that it matches the current character.

@private
@param {string} [c] - character to match
@returns {string} next character