ShapedLines are never wrapped, whereas WrappedLines are optionally wrapped if
they are associated with a wrap width. I tried to combine everything because
wrapping is inherently optional for the Text element, but we have a bunch of
APIs that don't make sense on a line that may wrap, so we need a distinct type
for that case.
This PR renames the `ThemeVariant` type to `Theme`.
This better reflects its purpose, as well as matches the same name as we
had before, which should make porting crates slightly easier.
Release Notes:
- N/A
