Migrate to dotenv parser

[midnight-wonderer]

Hello, I just discovered this project today; thanks.

The Issue

Some people are having trouble with Parser.php for some times. https://github.com/johnathanmiller/secure-env-php/pull/6 https://github.com/johnathanmiller/secure-env-php/issues/9 And I was very surprised to know that PCRE has been used for the parser.

How about we just use Dotenv just for the parser? This way you don't have to maintain the code here, leveraging on the community.

Notes

• phpunit was bumped one version to allow running on PHP 8
• ??= syntax required PHP 7.4; I bump the requirement as the result. Older PHP versions can still use the current release, since they are feature compatible.

What do you says?

Sound good?

P.S. I know that Laravel has a similar feature already; I use this on a CodeIgniter project.