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-includes/css/editor.css

@@ -2112,12 +2112,12 @@ i.mce-i-hr:before {
 	font-family: Tahoma, sans-serif;
 }
 
-/* @noflip */
+/* rtl:ignore */
 .mce-rtl .mce-flow-layout .mce-flow-layout-item > div {
 	direction: rtl;
 }
 
-/* @noflip */
+/* rtl:ignore */
 .mce-rtl .mce-listbox i.mce-caret {
 	left: 6px;
 }