Update CSS and RTL CSS style vendor prefixes per WPCS (#758)

* Add `@wordpress/browserslist-config`

* Add `autoprefixer`

* Add `postcss-cli`

* Update lock file

* Add `postcss.config.js` with browserslist and autoprefixer config

* Add npm scripts for updating vendor prefixes

* Update build instruction docs

* Update CSS and RTL CSS style vendor prefixes per WPCS

* Fix stylelint lint errors

CONTRIBUTING.md

@@ -59,4 +59,4 @@ Twenty Twenty uses [RTLCSS](https://rtlcss.com/) which allows to transform the
 
 ### Usage instructions
 
-- After making a change to a CSS, which affects RTL, run `npm run build:rtl` from within the theme directory to build `style-rtl.css` with your changes.
+- After making a change to a CSS, which affects RTL or Browser vendor prefixes, run `npm run build` from within the theme directory to build the CSS and RTL CSS files with your changes.

assets/css/editor-style-classic-rtl.css

@@ -65,8 +65,6 @@ body#tinymce.wp-editor { /* stylelint-disable-line no-duplicate-selectors */
 }
 
 body#tinymce.wp-editor * {
-	-webkit-box-sizing: border-box;
-	-moz-box-sizing: border-box;
 	box-sizing: border-box;
 	-webkit-font-smoothing: antialiased;
 }

assets/css/editor-style-classic.css

@@ -65,8 +65,6 @@ body#tinymce.wp-editor { /* stylelint-disable-line no-duplicate-selectors */
 }
 
 body#tinymce.wp-editor * {
-	-webkit-box-sizing: border-box;
-	-moz-box-sizing: border-box;
 	box-sizing: border-box;
 	-webkit-font-smoothing: antialiased;
 }