Travis/Composer: switch over to parallel linting of PHP files

[jrfnl]

Summary

This PR can be summarized in the following changelog entry:

• Use Parallel Linting

Relevant technical choices:

Composer

This installs two additional PHP packages in require-dev:

• php-parallel-lint which allows for linting PHP files in parallel (faster), as well as automatically recursively walking directories.
• php-console-highlighter which provides PHP code highlighting in the command line console, allowing the linter to display the results in a more meaningful manner.

It also adds a new lint script for use with Composer.

Travis

• Switch out the script part in the Travis script which did the linting the "old-fashioned" way to use the new Parallel linting option.
• Adjust the composer install command in the install section to always run.
• Note: for PHP "nightly" we need to ignore-platform-reqs for the time being as one of the PHPCS related dependencies does not allow for installation on PHP 8 yet. This has been fixed in the dependency, but there hasn't been a release yet containing the fix.

Includes removing an outdated comment in the Travis script.

Ref:

• https://github.com/php-parallel-lint/PHP-Parallel-Lint/releases/tag/v1.2.0
• https://github.com/php-parallel-lint/PHP-Console-Highlighter/releases/tag/v0.5

Test instructions

This PR can be tested by following these steps:

Testing this PR

• Check out this branch.
• Run composer install.
• Run composer lint & see it in action.
• Introduce a parse error in one of the files.
• Run composer lint & see the error being reported.
• Undo the parse error.

Also check a couple of the Travis builds to verify that the linting is running and passing.