Add Option to exclude post from Search

[burhandodhy]

Description of the Change

This PR adds the new option to exclude the certain post from Search.

Closes #253

Changelog Entry

Added - Option to exclude certain posts from Search

Credits

Props @burhandodhy @felipeelia @JakePT

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.

[JakePT]

@felipeelia @burhandodhy I've done a review of the front end in progress and pushed some tweaks:

1. I updated the auth_callback for the meta field to explicitly check permissions for the specific field and the specific post being edited.
2. I organised the JS into a file/folder for the feature, for consistency with most of the other scripts, and I updated the hook that it's enqueued with so that checking the current page isn't necessary.
3. I simplified the code for the checkbox plugin. The useState and useEffect calls weren't really necessary, and I adopted useSelect and useDispatch for handling the data. With previous implementation the "Update" button wasn't becoming enabled when the checkbox was changed for some reason, but this seems to have fixed that issue.
4. I updated the "Exclude from search" label to be sentence case, for consistency with the other checkboxes, such as "Stick to the top of the blog".

Let me known if you have any questions about the changes.

[felipeelia]

Before the release: let's create an article in the Help Center explaining how this works.

[burhandodhy]

LGTM