Hide typography units when the user is editing an e-mail template

[gabrielcaires]

Goal

The goal is to hide relative units from the typography panel when the user is editing an email template.

Acceptance Criteria

Scenario: Editing the email templates Given I have an email template customized When I select a block that is possible to use the typography panel Then I should only see "px" as an option.

Scenario: Editing any other post type Given I have a post type When I select a block that is possible to use the typography panel Then I should see all available options set by the theme.

Extra Notes

The scope was extracted from issue #6531 because we didn't find an official way to do it. A slack thread about it was started here p1677166826202419-slack-C45SNKV4Z