[BBT-54] Updates registered menu capability check

[chrishbite]

Description

Fixes: BBT-54

Updates the user capability used when adding the menu item for the Themer plugin.

Change Log

• Changes user capability used when registering the admin page from manage_options to edit_theme_options

Steps to test

1. Login as an Admin or Super Admin
2. The menu item of Settings > Theme Customiser should be visible
3. Click the above menu link, the Themer plugin should load
4. Log out
5. Login as a user with the role Editor
6. Confirm that Settings > Theme Customiser isn't visible
7. Attempt to visit */wp-admin/options-general.php?page=theme_settings
8. The Sorry, you are not allowed to access this page. message should be displayed

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules

[g-elwell]

Tested using admin and editor roles, works as expected 👍