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:

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

[swissspidy]

https://github.com/wearerequired/coding-standards requires an old version of wp-coding-standards/wpcs, so there are some issues on PHP 8.0

Then there are lint warnings for things like WordPress.Files.FileName.NotHyphenatedLowercase which should be disabled anyway.

[swissspidy]

Some PHPStan improvements incoming

[codecov[bot]]

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 :warning:
inc/CLI/LanguagePackCommand.php	0.00%	4 Missing :warning:
inc/CLI/ProjectCommand.php	40.00%	3 Missing :warning:
inc/Loader/Git.php	0.00%	3 Missing :warning:
inc/Loader/Mercurial.php	0.00%	3 Missing :warning:
inc/Loader/Subversion.php	0.00%	3 Missing :warning:
inc/ZipProvider.php	66.66%	2 Missing :warning:
inc/Export.php	93.33%	1 Missing :warning:
inc/Runner.php	50.00%	1 Missing :warning:
inc/WebhookHandler/GitHub.php	92.85%	1 Missing :warning:
... and 1 more

Additional details and impacted files

```diff @@ 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](https://app.codecov.io/gh/wearerequired/traduttore/pull/270/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=wearerequired) | Coverage Δ | | |---|---|---| | [feature](https://app.codecov.io/gh/wearerequired/traduttore/pull/270/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=wearerequired) | `13.57% <11.88%> (∅)` | | | [php](https://app.codecov.io/gh/wearerequired/traduttore/pull/270/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=wearerequired) | `79.76% <72.22%> (-1.45%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=wearerequired#carryforward-flags-in-the-pull-request-comment) to find out more.

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