Fix a bug when important has been set by another plugin

[elchininet]

This pull request fixes a bug related to some of the value directives and the important property of a declaration.

Some value directives assume that if the important property of a declaration is true, then raws.important has a value (the place of the directive). This is the case if the important is coming from the source code and the directive has been placed after !important, but that is not the case, for example, if important was set by a previous postcss plugin. In these cases, the important property is true but raws.important is undefined, because the CSS in those cases will be something like this (the directive will end before the important):

declaration: value /*directive*/ !important;

And in those cases the next error will be thrown:

Uncaught TypeError: Cannot read properties of undefined (reading 'substr')
    at /CWD/example.css:3:3
    at Object.action (/CWD/node_modules/rtlcss/lib/plugin.js:184:141)
    at /CWD/node_modules/rtlcss/lib/rtlcss.js:136:27
    at /CWD/node_modules/rtlcss/lib/util.js:259:37

Because of these types of conditions:

decl.important ? decl.raws.important.substr(9).trim() : ''
                                    ^
                                    |
                          /* important is undefined */

The solution is just to check for the existence of raws.important before trying to access to its string methods:

decl.important && decl.raws.important ? decl.raws.important.substr(9).trim() : ''

Closes: #329