Add changeset tooling

[stigi]

Change description

This PR adds tooling around managing change sets, by employing https://github.com/changesets/changesets

The source of truth for the project version now relies in package.json and is propagated via a script to all other places (Readme.md, swift sources).

I also imported the last changeset from the current version 2.0.0 as a baseline and added pending changesets for work that was done in #38 and that will go out in a major release.

A few things went into this to make it work:

• Added NPM configuration and the @changeset dependencies
• Initialized the changeset config with the default values
• Added a wrapper around yarn changeset that also calls the update-version.ts script to update the version accross the repo.
• Cocoapods is fine with loading the version directly via JSON import from package.json

Test Plan

Check out this branch, create an example changeset via yarn changeset or yarn changeset add. Bump the version via yarn changeset version. The version should be updated in package.json, Readme.md as well MagicBellClient.swift.

Also take a look at #42 which was created by the workflow that's part of this PR.

Type of Change

• Bug fix
• Feature
• Enhancement

[stigi]

@smeijer heads up that I did a small, but significant, change in 153640a

The workflow now invokes yarn changeset version instead of calling the JS function directly (see action code). This makes sure that the version numbers are correctly replaced everywhere.