Add support for the new @view-transition CSS rule (#4313)

With this release, esbuild now has improved support for pretty-printing and minifying the new @view-transition rule (which esbuild was previously unaware of):

/* Original code */
@view-transition {
  navigation: auto;
  types: check;
}

/* Old output */
@view-transition { navigation: auto; types: check; }

/* New output */
@view-transition {
  navigation: auto;
  types: check;
}

The new view transition feature provides a mechanism for creating animated transitions between documents in a multi-page app. You can read more about view transition rules here.

This change was contributed by @yisibl.

Trim CSS rules that will never match

The CSS minifier will now remove rules whose selectors contain :is() and :where() as those selectors will never match. These selectors can currently be automatically generated by esbuild when you give esbuild nonsensical input such as the following:

/* Original code */
div:before {
  color: green;
  &.foo {
    color: red;
  }
}

/* Old output (with --supported:nesting=false --minify) */
div:before{color:green}:is().foo{color:red}

/* New output (with --supported:nesting=false --minify) */
div:before{color:green}

This input is nonsensical because CSS nesting is (unfortunately) not supported inside of pseudo-elements such as :before. Currently esbuild generates a rule containing :is() in this case when you tell esbuild to transform nested CSS into non-nested CSS. I think it's reasonable to do that as it sort of helps explain what's going on (or at least indicates that something is wrong in the output). It shouldn't be present in minified code, however, so this release now strips it out.