[Enhancement] New heading style - H4

[henrikvoetmand]

• I have written a good issue

Describe the enhancement

In the design library (Sketch) we have the possibility to make a Section header in font size 16 - bold.
To align and support this in Kirby code, we could define a new heading size "H4" with font size 16 - bold.

Describe the solution you'd like

Define new text style "H4"

font-size: font-size("n");
font-weight: font-weight("bold");
CLASS: h4 or kirby-text-medium

Add this new text style to the "Fonts" page on the Cookbook

https://cookbook.kirby.design/#/home/showcase/fonts

Have you considered any alternatives?

Are there any additional context?

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