Add `selector-attribute-quotes` rule

[Stephen Edgar]

Via stylelint/stylelint#1137

https://make.wordpress.org/core/handbook/best-practices/coding-standards/css/#selectors

Correct:

input[type="text"] {
    line-height: 1.1;
}

Incorrect:

input[type=text] {
    line-height: 1.1;
}

input[type='text'] {
    line-height: 1.1;
}

[Stephen Edgar]

This is now merged, will be in the next stylelint release stylelint/stylelint#1506