Add early exit clauses to files with procedural code

[felixarntz]

Summary

This was flagged during plugin review of the "Dominant Color Images" submission. Any files with procedural code should have an early exit condition. Most typically the one based on whether ! defined( 'ABSPATH' ), but another relevant constant can used as well. For example, in the files where we already check for a Performance Lab specific constant on top of the file, we don't have to add to or update those files.

Keep in mind that this is only required for files with procedural code, i.e. not files that only contain a class or similar construct.

There are varying opinions on this approach, but given it's requested by the plugin review team and is not a big deal to add, let's follow the pattern.

Checklist

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