feat(styles): one stylesheet source for react and svelte (#3350)

* feat(styles): create stylesheets for react and svelte based on one source

* refactor(styling): cleanup

* refactor(postcss): share a config

* refactor(postcss): replace env hack with env file

packages/svelte/src/styles/base.css

@@ -0,0 +1,3 @@
+/* this gets exported as style.css and can be used for the default theming */
+@import '../../../system/src/styles/init.css';
+@import '../../../system/src/styles/base.css';