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
Built from https://develop.svn.wordpress.org/trunk@31573


git-svn-id: http://core.svn.wordpress.org/trunk@31554 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/css/themes-rtl.css

@@ -178,7 +178,7 @@
 	background: rgba(0,0,0,0.7);
 	color: #fff;
 	font-size: 15px;
-	text-shadow: 0 -1px 0 rgba(0,0,0,0.6);
+	text-shadow: 0 1px 0 rgba(0,0,0,0.6);
 	-webkit-font-smoothing: antialiased;
 	font-weight: 600;
 	padding: 15px 12px;
@@ -1524,6 +1524,7 @@ body.full-overlay-active {
 	text-decoration: none !important;
 }
 
+/* rtl:ignore */
 .wp-full-overlay.collapsed .collapse-sidebar-arrow:before,
 .rtl .wp-full-overlay .collapse-sidebar-arrow:before {
 	-webkit-transform: rotate(180deg);