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/lib/components/Attribution/Attribution.svelte

@@ -24,17 +24,3 @@
     </a>
   </Panel>
 {/if}
-
-<style>
-  :global(.svelte-flow__panel.svelte-flow__attribution) {
-    font-size: 10px;
-    background: rgba(255, 255, 255, 0.5);
-    padding: 2px 3px;
-    margin: 0;
-  }
-
-  :global(.svelte-flow__attribution) a {
-    text-decoration: none;
-    color: #999;
-  }
-</style>