add support for dimensions relative to font size

## What problem does this solve or what need does it fill?

To help with responsive design, dimensions relative to font size are useful

## What solution would you like?

Add support for `em` and `rem`. Additionally, `ex` and `ch` exist but are less useful

## What alternative(s) have you considered?

None

## Additional context

Unit|Description
--|--
em | Relative to the font-size of the element (2em means 2 times the size of the current font)
rem | Relative to font-size of the root element
ex | Relative to the x-height of the current font (rarely used)
ch | Relative to the width of the "0" (zero)

https://www.w3schools.com/cssref/css_units.php

No files or tests are named in the issue. Start by locating the dimension and unit definitions, then inspect existing unit tests and examples for how absolute and relative dimensions are represented; done means the requested font-relative units are accepted and covered by tests, with their documented behavior reflected in the layout results.

Written by the indexing model from the issue text.