Add support for "isolate" value in "unicode-bidi"

[Krinkle]

The following should afaik be parsed as not containing any unknown properties:

.mw-example {
    unicode-bidi: -moz-isolate;
    unicode-bidi: -webkit-isolate;
    unicode-bidi: isolate;
}

Right now in e.g. csslint, it results in:

• Expected (normal | embed | bidi-override | inherit) but found '-moz-isolate'.
• Expected (normal | embed | bidi-override | inherit) but found '-webkit-isolate'.
• Expected (normal | embed | bidi-override | inherit) but found 'isolate'.

See also:

• CSS/unicode-bidi on MDN
• Issue Add support for "isolate" value in "unicode-bidi" csslint#436