Restrict plugin to the post editor

[ingeniumed]

Description

• This PR is meant to restrict the plugin to only the post editor, when a post/page is edited or created. Anywhere else it will be disabled until we properly add support for that in Allow restricting the rules based on the page #80. This will fix Governance rules interfering with the Widget Editor #79, and ensure that we don't inadvertently repeat this on other pages.
• This will also remove the number formatter library so the plugin is playground compatible. I've also added in a playground link.

Steps to Test

1. Set up a restricted rules file
2. Load the twentytwentyone theme
3. Go to the widgets screen and ensure the rules don't load
4. Go and edit a post/page, or create a new one and ensure the rules load
5. Go to the site editor and ensure the rules don't load

[alecgeatches]

✅ @ingeniumed this works well! Tested with a widget-enabled theme (twentytwentyone as suggested) and this branch fixed the PR. Also tested the ordinal changes which work fine. Thank you!