Add a mode indicator for vim

This is the second most common remaining complaint (after :w not
working).

Fixes: zed-industries/community#409

styles/src/style_tree/status_bar.ts

@@ -27,6 +27,7 @@ export default function status_bar(): any {
         },
         border: border(layer, { top: true, overlay: true }),
         cursor_position: text(layer, "sans", "variant"),
+        vim_mode: text(layer, "sans", "variant"),
         active_language: interactive({
             base: {
                 padding: { left: 6, right: 6 },