Allow setting margin-bottom on custom typography targets

[frzsombor]

Description

Describe your feature request.

As the title says, I would like to set a "margin-bottom" for custom typography selectors too.
The reason is, many times I work with a design, the designer uses H1, H2, etc. STYLES differently, than what is required for good SEO (for example designer uses H3 style where I have to insert an actual H2 tag). For these situations I started using custom selectors for defining Typography defaults like h1, .text-like-h1, h2, .text-like-h2, etc. However, I can not set a margin-bottom, as it is not supported for custom selectors, only a few others. However, this only requires a very simple modification as class-typography.php already handles margin-bottom for all elements.

References

https://github.com/tomusborne/generatepress/blob/3.4.0/src/customizer-controls/font-manager/utils.js#L45-L47
https://github.com/tomusborne/generatepress/blob/3.4.0/inc/class-typography.php#L176-L179