[BUG] `inset` not overridden correctly in fallbacks

[jamesnw]

Describe the bug
Given

 inset: auto;
 top: anchor(bottom);
 left: anchor(right);

the position-try fallback is created as:

  left:var(--anchor-15OcXs-HWJ-Q)!important;
  top:revert!important;
  bottom:var(--anchor-NFBDsMuRrZSb)!important;
  inset:auto!important;

The inset is added after the others, which cascades to override the other values. In addition, top conflicts, and should not be set.

To Reproduce
Create a target with position-try-fallbacks and inset: auto.

Expected behavior
The inset shorthand should work in combination with the other values as expected.

Screenshots
If applicable, add screenshots to help explain your problem.

Browsers tested on
Polyfill v0.5.1