Add support for `em` TextStyles

### Use case

It is common practice to have larger letter spacing depend on the size of the text.

Flutter only supports setting a letter spacing in terms of pixels.
This makes it much more challenging to re-use themes. A letter-spacing must be set for every font size.

### Proposal

Add a property called `letterSpacingScale` to `TextStyler` which will used to set the letterSpacing.

This idea is found in CSS where it is defined using `em`.

(Maybe we should add `rem` too! This would be really neat for Flutter Web)