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/about.css

@@ -263,6 +263,7 @@
 	opacity: 0;
 }
 
+/* rtl:ignore */
 .about-wrap .dfw-container .overlay-image.from-left {
 	-webkit-transform: translate3d(-100%, 0, 0);
 	transform: translate3d(-100%, 0, 0);
@@ -296,7 +297,7 @@
 .about-wrap .feature-list svg {
 	float: left;
 	clear: left;
-	margin: 15px 15px 0 0 ;
+	margin: 15px 15px 0 0;
 	height: 90px;
 	width: 90px;
 	background-color: #cccccc;