refactor(jqLite): refactor the attr method

The attr method was refactored to be divided into setter & getter parts
and to handle boolean attributes in each one of them separately instead of
dividing into boolean & non-boolean ones and then handling setter & getter
in both of them. This is because handling boolean & non-boolean attributes
has common parts; in particular handling of the `null` value or using
getAttribute to get the value in the getter.