Update wp-config.php regex to handle strict types declaration

[andyexeter]

Hey there,

Currently, when this plugin adds the WP_CACHE constant to wp-config.php it adds it right at the top of the file, just below the <?php or <? tag.

This is causing fatal errors for us because we use strict types across the board:

<?php

declare(strict_types=1);

The constant is being added above the declare statement, causing the following error:

PHP Fatal error:  strict_types declaration must be the very first statement in the script in /app/public/wp-config.php on line 5

This PR updates the regex used to place the constant below the declare statement if it is present.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 1.52%. Comparing base (5f6f522) to head (eb5176a).
⚠️ Report is 25 commits behind head on master.

Files with missing lines	Patch %	Lines
PgCache_Environment.php	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             master   #1250   +/-   ##
========================================
  Coverage      1.52%   1.52%           
  Complexity    19723   19723           
========================================
  Files           634     634           
  Lines         98658   98658           
========================================
  Hits           1509    1509           
  Misses        97149   97149           

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