This is an unusually large commit that should untangle some of the
additional Ruby code run by the Vagrantfile, before the `Vagrant.configure` block is run.

- Moving functionality related to VVV that is run before the
  `Vagrant.configure` block into a separate Ruby module and classes
- The resulting classes and methods now follow the Ruby Style Guide
- Implementing simple and reasonable Rubocop ruleset
- Adding a Rubocop Github action
- Adding a `.ruby-version` file indicating the Ruby version currently
  used by Vagrant (2.7.6)
- Few if any functional changes have been introduced intentionally
- Further code cleanup

The following classes were created:

- `VVV::Info` - Fetches info on the platform and environment
- `VVV::Config` - Reads and processes the config file
- `VVV::SplashScreens` - Displays splash screens
- `VVV::Bootstrap` - Indicates when to display splash screens
- `VVV::Migrate` - Handles config file and database migrations

Caveats:

- This covers what happens before `Vagrant.configure`
- The cleanup is required to keep the Ruby code maintainable and to
  facilitate further development.

Bugfixes:
- The non-functionning `Vagrant.has_plugin?` may be replaced by `VVV::Info.vagrant_local_plugins` and `VVV::Info.vagrant_global_plugins`

To be tested:

[ ] Config migration
[ ] Database migration
[ ] Splash screens and
[ ] Config defaults
[ ] Config processing and backwards-compatibility
[ ] Github action for Rubocop
[ ] Different host platforms and architectures