Add support for Dotenv v3, v4, and v5

[mcaskill]

Checklist:

• [x] I've read the Contributing page.
• [ ] I've created an issue and referenced it here.
• [x] My code is tested.
• [x] My code follows the WordPress code style.
• [x] My code has proper inline documentation.

Description

Adds support for Dotenv v5 while also keeping support for v4 and restoring support for v3.

This allows older projects that might be stuck on an older version of Dotenv to continue to benefit from updates to this Composer plugin.

How has this been tested?

I'm testing this on a client project that uses Advanced Custom Fields Pro, Gravity Forms, and Polylang Pro.

Types of changes

Added

• Method Installer::loadDotenv() to resolve and instantiate the currently available version of the Dotenv class.
• Class AbstractPlugin as basis for all supported WP plugins. The class provides a get() method which serves as a wrapper of getenv() that will throw an exception if the requested environment variable is missing.
• Class MissingEnvException in case an environment variable is missing.

Changed

• WP plugin classes to extend new AbstractPlugin and use its get() method instead of directly using getenv() function.

[junaidbhura]

Thanks @mcaskill ! As always, much appreciated!

Let me take a deeper look and get back to you!

[junaidbhura]

Hey @mcaskill can we limit this PR to just the changes in Installer.php and composer.json?

I'm looking to do a big refactor of this plugin in the coming months, which will implement an Abstract class, etc.

[mcaskill]

Sorry for the delay and thanks for the feedback.

[junaidbhura]

Thanks @mcaskill really appreciate it as always 👍