fix: Make package importable for type="module" projects

[susnux]

Explicitly set the type of the dist files by changing the file extension to mjs or cjs respectively.

Without this fix, you get following error when importing from this package:

SyntaxError: Named export 'emit' not found. The requested module '@nextcloud/event-bus' is a CommonJS module, which may not support all module.exports as named exports.
CommonJS modules can always be imported via the default export, for example using:

import pkg from '@nextcloud/event-bus';
const { subscribe: r, emit: a, unsubscribe: c } = pkg;

This happens because the type is not set, so node assumes commonjs and will interpret every .js file as commonjs file regardless of the exports section.

[skjnldsv]

Sure