Allow custom labels to be used in the replacement variable editor

[jcomack]

Summary

This PR can be summarized in the following changelog entry:

• [replacement-variable-editor] Adds possibility to pass alternative labels for the SEO title and Meta description fields in the SettingsSnippetEditor.

Relevant technical choices:

• This PR introduces a change that will be used in Yoast SEO. Testing this change can be done based on the PR that will be made in the plugin and this PR only 'enables' the possibility.

Test instructions

This PR can be tested by following these steps:

• Checkout this branch and build accordingly.
• Use yarn link-monorepo to link the entire monorepo to your local Yoast SEO installation.
• Ensure your Yoast SEO installation is fully built (composer install, yarn, grunt build).
• Navigate to SEO -> Search Appearance -> Content Types.
• See that nothing has changed in the forms and the usual SEO title and Meta description labels are still present. This means that the default values are properly working.

Impact check

• This PR affects the following parts of the plugin, which may require extra testing:
  *

UI changes

• This PR changes the UI in the plugin. I have added the 'UI change' label to this PR.

Quality assurance

• I have tested this code to the best of my abilities
• I have added unittests to verify the code works as intended

Required for Yoast/wordpress-seo#16806

[enricobattocchi]

CR OK!

[afercia]

Acceptance 👍