feat: add missing index.ts (#886)

The `package.json` has ` "exports": "./dist/index.js"`, but no actual `index.ts` or `index.js`
exists.

This commit adds that file, in the same format as the one in `plugin-plugins`:
https://github.com/oclif/plugin-plugins/blob/main/src/index.ts

src/index.ts

@@ -0,0 +1,10 @@
+import CommandsUpdate from './commands/update.js'
+import {init} from './hooks/init.js'
+
+export const commands = {
+  update: CommandsUpdate,
+}
+
+export const hooks = {
+  init,
+}