Add wp-env and utilize for running PHPUnit locally

[westonruter]

This incorporates work from @delawski in ampproject/amp-wp#6802, as well as prior work from @pierlon in ampproject/amp-wp#5201.

In order to run tests with Xdebug in IntelliJ/PhpStorm in the included wp-env environment:

0. Open Preferences and go to Languages & Frameworks > PHP.

1. Open Frameworks and under WordPress, toggle on to Enable WordPress integration.

For the WordPress installation path, obtain the root via npm run wp-env install-path and then append /wordpress-develop/src. For example:

/Users/westonruter/.wp-env/e6c93f608f26ae7e0e7a5f1fa29a6a3e/wordpress-develop/src

(The hash will be different on your system.)

Then add this path without /src to the PHP Include Path:

/Users/westonruter/.wp-env/e6c93f608f26ae7e0e7a5f1fa29a6a3e/wordpress-develop

2. Then open the Servers section and add a pwa server with the host set to host.docker.internal and port to 8888, with the project root mapping to /var/www/html/wp-content/plugins/pwa:

Also under Include path add the path mappings from /src and /tests from the previous step to map to /var/www/html/ and /var/www/wordpress-develop/tests respectively:

3. Open the Debug section, and configure the External connections and Xdebug as following (which it may be by default):

[codecov-commenter]

Codecov Report

Merging #778 (5cebfbd) into develop (7ade9ff) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             develop     #778   +/-   ##
==========================================
  Coverage      19.01%   19.01%           
  Complexity       346      346           
==========================================
  Files             57       57           
  Lines           2324     2324           
==========================================
  Hits             442      442           
  Misses          1882     1882           

Flag	Coverage Δ
php	19.01% <ø> (ø)
unit	19.01% <ø> (ø)

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[westonruter]

I always get tripped up on configuring Xdebug with PhpStorm/IntelliJ, so if anyone has improvements to what I've outlined above I'd appreciate it. I want to make sure the best most-streamlined practice is followed. The notes are based on configuring a plugin-specific project, where the project root is the PWA repo root.