PayPal js files loaded on non PayPal pages (3341)

[stracker-phil]

Problem:

Some plugin assets (JS files) were loaded on WooCommerce settings pages that did not require them.

Checks

• cart-paylater-block-inserter.js
  • Reported to load on any "WooCommerce → Settings" tab, but not reproducible
  • Only loaded on (admin) pages that also have the Block Editor, via enqueue_block_editor_assets
• settings.js ← optimized
  • Previously loaded on the "WooCommerce → Settings → Payments" tab
  • Now only loaded on "WooCommerce → Settings → Payments → PayPal" (and related, like "Advanced Card Processing")
  • Addressed in line 171 of the commit
• Besides the plugin settings tabs ("WooCommerce → Settings → Payments → PayPal"), only two other pages correctly enqueue plugin scripts:
  • Two assets are loaded on the "Order details" page
  • Some assets are loaded in Block Editor, when editing a page/post

Further Cleanup:

This PR also does minor cleanup of the relevant PHP class

1. Remove unnecessary trailingslashit() calls in the register() method
2. Change return type of the enqueue() method from bool to void - it's only used as an action callback