chore: add changeset for #80 vite-plugin HMR fix

Author: bennypowers

.changeset/fix-vite-plugin-hmr.md

@@ -0,0 +1,7 @@
+---
+"@pwrs/vite-plugin-lit-css": patch
+---
+
+fix: enable HMR for vite-plugin-lit-css
+
+Add `addWatchFile` in the `load` hook so Vite watches the underlying CSS files for changes, and add `handleHotUpdate` hook to invalidate virtual modules when their source CSS files change.