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/system/src/styles/base.css

@@ -0,0 +1,33 @@
+.xy-flow {
+  --node-border-default: 1px solid #bbb;
+  --node-border-selected-default: 1px solid #555;
+
+  --handle-background-color-default: #333;
+
+  --selection-background-color-default: rgba(150, 150, 180, 0.1);
+  --selection-border-default: 1px dotted rgba(155, 155, 155, 0.8);
+}
+
+.xy-flow__handle {
+  background-color: var(--handle-background-color, var(--handle-background-color-default));
+}
+
+.xy-flow__node-input,
+.xy-flow__node-default,
+.xy-flow__node-output,
+.xy-flow__node-group {
+  border: var(--node-border, var(--node-border-default));
+
+  &.selected,
+  &:focus,
+  &:focus-visible {
+    outline: none;
+    border: var(--node-border-selected, var(--node-border-selected-default));
+  }
+}
+
+.xy-flow__nodesselection-rect,
+.xy-flow__selection {
+  background: var(--selection-background-color, var(--selection-background-color-default));
+  border: var(--selection-border, var(--selection-border-default));
+}