move shim to new package

packages/shim/src/css-overrides.js

@@ -0,0 +1,9 @@
+// Injects a link to the CSS overrides stylesheet served from /assets/overrides.css.
+
+export function installCssOverrides() {
+  const link = document.createElement("link");
+  link.rel = "stylesheet";
+  link.href = "/assets/overrides.css";
+  link.setAttribute("data-ignis", "css-overrides");
+  document.head.appendChild(link);
+}