Fix Windows-unfriendly theme.scss loader rule

The webpack module loader rule for theme.scss files contained a
unix-style path separator that resulted in theme.scss not being matched
with the ExtractTextPlugin and processed by SASS. Instead, it was being
treated as a JS import, and our stub `core-blocks/separator/theme.scss`
file happens to also parse as JS.  This meant there was no build error
but also no `build/core-blocks/theme.css` file. This commit simply
removes the offending separator because it is unnecessary.

webpack.config.js

@@ -243,7 +243,7 @@ const config = {
 				use: editBlocksCSSPlugin.extract( extractConfig ),
 			},
 			{
-				test: /\/theme\.s?css$/,
+				test: /theme\.s?css$/,
 				include: [
 					/core-blocks/,
 				],