skew() is apparently legacy and our -ms variant might be wrong

Created by: cvrebert

Regarding https://github.com/twbs/bootstrap/blob/82e2f87363d960b647bc1e97bf53bbb229b0e703/less/mixins/vendor-prefixes.less#L133

We're currently using transform: skew(@x, @y);. According to MDN (emphasis mine):

Note: The skew() function was present in early drafts. It has been removed but is still present in some implementations. Do not use it.

Similarly, the current spec says (emphasis mine):

Note that the behavior of skew() is different from multiplying skewX() with skewY(). Implementations must support this function for compatibility with legacy content.

So, I'm thinking that we should stop using skew(x, y)?

Relatedly, we're currently using -ms-transform: skewX(@x) skewY(@y), which based on that spec note sounds like it might not be the correct equivalent. Some testing is probably warranted.