Accept flex-flow's two values in either order

[jhaygood86]

Problem

flex-flow: row wrap;   /* accepted */
flex-flow: wrap row;   /* dropped */

flex-flow is <'flex-direction'> || <'flex-wrap'> (CSS Flexbox 1 §5.1). The double bar means the operands may appear in either order, but the two-value alternative is built with WithOrder, which is the OrderedOptionsConverter:

return directionConverter
      .Or(wrapConverter)
      .Or(WithOrder(directionConverter, wrapConverter));   // <-- requires direction first

So a reversed pair fails to convert and the declaration is discarded.

Fix

Use WithAny (the UnorderedOptionsConverter) for the pair.

|| still permits at most one occurrence of each operand, and that is preserved: row row, wrap wrap, row column, bogus wrap and wrap bogus all remain invalid. Reversed input normalizes to the canonical direction-then-wrap value, so flex-flow: wrap row yields row wrap.

I checked the other WithOrder call sites — flex, border-radius, font, background, border-image, cursor, counter-increment/-reset, transform-origin, the ratio and background-position/size compositions — and they all model genuinely ordered grammars. flex-flow was the only || implemented as ordered.

Known gap, left alone

Reconstructing the shorthand from its longhands still yields only the direction — flex-flow: row wrap read back via StyleDeclaration.FlexFlow gives row. That behaviour predates this change (it reproduces identically on master) and is out of scope here.

It isn't a one-line follow-up either: Or picks the first alternative that succeeds in Construct as well as Convert, so moving the pair ahead of the single-value alternatives fixes row wrap but regresses flex-flow: row to serialize as row initial, because the unset operand constructs the literal initial sentinel that ShorthandProperty.Export writes. Fixing it properly means suppressing that sentinel during re-serialization, which is a separate change.

Tests

14 theory cases in PropertyTests/FlexPropertyTests.cs: both orders for three direction/wrap pairs (asserting the normalized value and the two longhands), single values, and the invalid combinations.

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