Accept revert/revert-layer/unset on every property

[jhaygood86]

Problem

revert, revert-layer and unset are rejected on most properties, while inherit and initial work:

color: unset;              /* rejected */
display: revert;           /* rejected */
white-space: revert-layer; /* rejected */
color: inherit;            /* fine */

All five are CSS-wide keywords, valid on every property (CSS Cascade 4 §7.3; revert-layer is Cascade 5).

OrGlobalValue already accepts all five, but the widely-used OrDefault accepts only inherit and initial:

public static IValueConverter OrDefault(this IValueConverter primary)
{
    return primary.OrInherit().Or(Keywords.Initial);   // no revert / revert-layer / unset
}

so any property converter built with OrDefault (the majority) rejects the other three.

Fix

Route OrDefault through OrGlobalValue, and extend the defaulted overload (OrDefault<T>) with the same three keywords. The Keywords.Revert/RevertLayer/Unset constants already exist.

Tests

15 cases in PropertyTests/GlobalKeywordPropertyTests.cs: all five keywords across color, display, text-align, float, white-space, font-weight, line-height; plus misspellings (reverts, un-set, revertlayer) that must stay rejected, confirming the accepted set widened without loosening the grammar.

12 fail on master. The full suite (1263 existing tests) stays green, and all seven target frameworks build with no new warnings.