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/routes/overview/CustomNode.svelte

@@ -0,0 +1,27 @@
+<script lang="ts">
+  import { Handle, Position, type NodeProps } from '../../lib/index';
+
+  type $$Props = NodeProps;
+
+  export let data: { label: string } = { label: 'Node' };
+  export let xPos: number = 0;
+  export let yPos: number = 0;
+</script>
+
+<div class="custom">
+  <div>{data.label}</div>
+  <div>{~~xPos}, {~~yPos}</div>
+
+  <Handle type="target" position={Position.Top} />
+
+  <Handle type="source" position={Position.Bottom} style="left: 10%;" id="a" />
+  <Handle type="source" position={Position.Bottom} id="b" />
+  <Handle type="source" position={Position.Bottom} style="left: 90%;" id="c" />
+</div>
+
+<style>
+  .custom {
+    background: #ffcc00;
+    padding: 10px;
+  }
+</style>