PHPCS: Refresh PHPCS config

 - Add comments and links for those not familiar with PHPCS configuration file.
 - Add a default starting point for scanning. Can be overridden in the command line.
 - Exclude `vendor/` directory, since this is third-party code.
 - Add options to show violation code and progress.
 - Add basepath option so reports don't include full absolute path to file.
 - Add parallel processing for quicker reporting of PHPCS.
 - Limit scans to PHP files (copied from preference in Makefile). PHPCS 4 will remove the JS tokenizer anyway.
 - Add PHPCompatibilityWP ruleset, with a testVersion of 7.3 and above.
 - Change from deprecated / removed `WordPress-VIP` ruleset in WordPress Coding Standards, to `WordPress-VIP-Go` ruleset from VIP Coding Standards.

See #1032.

phpcs.xml

@@ -1,8 +1,15 @@
 <?xml version="1.0"?>
 <ruleset name="VIP-Go-mu-plugins">
-	<description>VIP Go mu-plugins</description>
+	<description>Custom ruleset for VIP Go mu-plugins</description>
 
-	<config name="ignore_warnings_on_exit">true</config>
+	<!-- For help in understanding this file: https://github.com/squizlabs/PHP_CodeSniffer/wiki/Annotated-ruleset.xml -->
+	<!-- For help in using PHPCS: https://github.com/squizlabs/PHP_CodeSniffer/wiki/Usage -->
+
+	<!-- What to scan -->
+	<file>.</file>
+
+	<!-- Ignoring Files and Folders:
+		https://github.com/squizlabs/PHP_CodeSniffer/wiki/Advanced-Usage#ignoring-files-and-folders -->
 
 	<!-- Submodules - these should have their own PHPCS configurations -->
 	<exclude-pattern>advanced-post-cache/*</exclude-pattern>
@@ -25,12 +32,32 @@
 	<exclude-pattern>drop-ins/*</exclude-pattern>
 	<exclude-pattern>shared-plugins/*</exclude-pattern>
 	<exclude-pattern>vaultpress/*</exclude-pattern>
+	<exclude-pattern>vendor/*</exclude-pattern>
 	<exclude-pattern>wordpress-importer/*</exclude-pattern>
 	<exclude-pattern>wp-cron-control/*</exclude-pattern>
 
 	<!-- Other files (borrowed and modified from a Symfony package) -->
 	<exclude-pattern>lib/proxy/ip-utils.php</exclude-pattern>
 	<exclude-pattern>tests/test-lib-proxy-ip-utils.php</exclude-pattern>
 
-	<rule ref="WordPress-VIP"/>
+	<!-- How to scan -->
+	<!-- Show sniff and progress -->
+	<arg value="sp"/>
+	<!-- Strip the file paths down to the relevant bit -->
+	<arg name="basepath" value="."/>
+	<!-- Enables parallel processing when available for faster results. -->
+	<arg name="parallel" value="8"/>
+	<!-- Limit to PHP files -->
+	<arg name="extensions" value="php"/>
+
+	<config name="ignore_warnings_on_exit">true</config>
+
+	<!-- Rules: Check PHP version compatibility - see
+		https://github.com/PHPCompatibility/PHPCompatibilityWP -->
+	<rule ref="PHPCompatibilityWP"/>
+	<!-- For help in understanding this testVersion:
+		https://github.com/PHPCompatibility/PHPCompatibility#sniffing-your-code-for-compatibility-with-specific-php-versions -->
+	<config name="testVersion" value="7.3-"/>
+
+	<rule ref="WordPress-VIP-Go"/>
 </ruleset>