refactor: include `composer.json` in plugin distribution package

Following WordPress Plugin Review Team recommendation, allow original
`composer.json` file to be available in distribution package.

The reasoning of WP Team:

> We noticed that your plugin is using Composer to handle library
> dependencies, that's great as it will help maintaining and updating
> your plugin in the future while avoiding collisions with other plugins
> that are using same libraries. The composer.json file describes the
> dependencies of your project and may contain other metadata as
> well. It's a small file that typically can be found in the top-most
> directory of your plugin. As one of the strengths of open source is
> the ability to review, observe, and adapt code, we would like to ask
> you to include that file in your plugin, even if it is only used for
> development purposes. This will allow others to exercise the open
> source freedoms from which we all benefit.

It's best to comply with those recommendation to avoid bounce-backs in
the future during free plugin review, considering that `composer.json`
itself will not cause any harm and will not reveal any disclosed
information about our plugin architecture or company internals.

Following, I don't think it is necessary to include `composer.lock` file,
which holds the information of exact version of installed dependencies.

Signed-off-by: Bart Jaskulski <bjaskulski@protonmail.com>