Cleanup, add PHP file support

[swissspidy]

Description

• Require PHP 7.4
• Requires GlotPress 4.0
• Add support for PHP translation files
• Improve tests & logic for webhook handlers
• Improve code quality, enforcing PHPStan level 9
• Verify SHA 256 signatures for GitHub webhooks instead of SHA1
• Fixes Behat code coverage configuration

Fixes #237
Fixes #256
Fixes #257
Fixes #241
Fixes #123

How has this been tested?

Unit tests all pass.

Screenshots

N/A

Types of changes

Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code has proper inline documentation.

[swissspidy]

Some PHPStan improvements incoming

[codecov]

Codecov Report

Attention: Patch coverage is 71.28713% with 29 lines in your changes are missing coverage. Please review.

Project coverage is 72.45%. Comparing base (a9faec1) to head (542f140).
Report is 1 commits behind head on master.

Files	Patch %	Lines
inc/Plugin.php	36.36%	7 Missing ⚠️
inc/CLI/LanguagePackCommand.php	0.00%	4 Missing ⚠️
inc/CLI/ProjectCommand.php	40.00%	3 Missing ⚠️
inc/Loader/Git.php	0.00%	3 Missing ⚠️
inc/Loader/Mercurial.php	0.00%	3 Missing ⚠️
inc/Loader/Subversion.php	0.00%	3 Missing ⚠️
inc/ZipProvider.php	66.66%	2 Missing ⚠️
inc/Export.php	93.33%	1 Missing ⚠️
inc/Runner.php	50.00%	1 Missing ⚠️
inc/WebhookHandler/GitHub.php	92.85%	1 Missing ⚠️
... and 1 more

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #270      +/-   ##
============================================
- Coverage     81.21%   72.45%   -8.76%     
+ Complexity      369      350      -19     
============================================
  Files            24       29       +5     
  Lines           942     1267     +325     
============================================
+ Hits            765      918     +153     
- Misses          177      349     +172     

Flag	Coverage Δ
feature	13.57% <11.88%> (∅)
php	79.76% <72.22%> (-1.45%)	⬇️

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[ocean90]

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