Add missing index.ts

[mattwebbio]

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

This file is necessary for extending functionality of the <cli> update command by extending it. Prior to the ESM migration, this could have been done like so:

import SuperUpdate from '@oclif/plugin-update/dist/commands/update'

export default class Update extends SuperUpdate {

With the change in this commit, this would now be done like so:

import {commands} from '@oclif/plugin-update'

export default class Update extends commands['update'] {

[mdonnalley]

Thanks for the contribution @mattwebbio