YoastCS rules: enforce fully qualified global functions and constants

Related to 303

Impact on Yoast packages:

| Plugin/Tool       | Errors/Warnings |
|-------------------|-----------------|
| PHPUnit Polyfills | --
| WP Test Utils     | --
| YoastCS           | --
| WHIP              | --
| Yoast Test Helper | --
| Duplicate Post    | --
| Yst ACF           | --
| Yst WooCommerce   | 1
| Yst News          | 2
| Yst Local         | --
| Yst Video         | --
| Yst Premium       | 18
| Yst Free          | 46

Note: this rule was previously already "silently" enforced via clean-up sweeps.

Yoast/ruleset.xml

@@ -307,6 +307,10 @@
 	<rule ref="Universal.UseStatements.DisallowUseConst"/>
 	<rule ref="Universal.UseStatements.DisallowUseFunction"/>
 
+	<!-- CS: For global functions/constants: use fully qualified inline names. -->
+	<rule ref="SlevomatCodingStandard.Namespaces.FullyQualifiedGlobalConstants"/>
+	<rule ref="SlevomatCodingStandard.Namespaces.FullyQualifiedGlobalFunctions"/>
+
 
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