RTL CSS generation: Switch from CSSJanus to RTLCSS.

CSSJanus (introduced in [26107]), we had a great time with you, but sadly you don't like our fancy CSS.

RTLCSS is a framework for converting CSS from LTR to RTL, same as CSSJanus, with support for more CSS properties like `transform`, `transition` or multiple box and text shadows.

Changes:
* devDependencies: Remove `grunt-cssjanus`, add `grunt-rtlcss`.
* RTLCSS uses `/* rtl:ignore */` to ignore a rule, switch existing `/* @noflip */` to the new directive.
* RTLCSS supports the `transform` property, means we can remove some ignore rules.
* RTLCSS supports string maps for custom replace rules. This commit includes a rule `import-rtl-stylesheet` which replaces ".css" with "-rtl.css" in URLs.

Notes for core development:
* The file generation task is still `grunt rtl`.
* If you have used `grunt cssjanus` before, use `grunt rtlcss` now.
* Remember the new directive `/* rtl:ignore */`.

fixes #31332.

Build: https://build.trac.wordpress.org/changeset/31554

git-svn-id: https://develop.svn.wordpress.org/trunk@31573 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/css/edit.css

@@ -1310,7 +1310,6 @@ table.links-table {
 	transition-timing-function: ease-in-out;
 }
 
-/* @noflip */
 .focus-on #adminmenuback,
 .focus-on #adminmenuwrap {
 	-webkit-transform: translateX( -100% );
@@ -1318,14 +1317,6 @@ table.links-table {
 	transform: translateX( -100% );
 }
 
-/* @noflip */
-.rtl.focus-on #adminmenuback,
-.rtl.focus-on #adminmenuwrap {
-	-webkit-transform: translateX( 100% );
-	-ms-transform: translateX( 100% );
-	transform: translateX( 100% );
-}
-
 .focus-off #adminmenuback,
 .focus-off #adminmenuwrap {
 	-webkit-transform: translateX( 0 );