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

package.json

@@ -22,10 +22,10 @@
     "grunt-contrib-qunit": "~0.5.2",
     "grunt-contrib-uglify": "~0.6.0",
     "grunt-contrib-watch": "~0.6.1",
-    "grunt-cssjanus": "~0.2.4",
     "grunt-jsvalidate": "~0.2.2",
     "grunt-legacy-util": "^0.2.0",
     "grunt-patch-wordpress": "~0.3.0",
+    "grunt-rtlcss": "~1.5.1",
     "grunt-sass": "~0.16.0",
     "matchdep": "~0.3.0"
   }