Allow multiple position-area declarations for a single selector

[jgerigmeyer]

Given:

.target {
  position-area: start;
}

.target {
  position-area: end;
}

Before, whichever one we parse last would be applied. Now this stores all of them, and allows the cascade to determine which one should be applied. (Or at least I think so -- some quick functional testing seemed to confirm that this works as expected.)

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[jamesnw]

Excellent cleanup. Thanks!