Add support for <strong> elements

[daymxn]

You can learn about what exactly this element is, and what sort of props it has here: click me!
What we want:

• builder variants for each tag
• element variants for each tag
• All props defined in the above link are supported
• It follows standard practices for this repo
• It has proper KDoc annotation, in a similar style to the others
• The code is idiomatic and not verbose