Multi-element are now generic over any drawable child, which can be converted
into an element.
Co-Authored-By: Nathan Sobo <nathan@zed.dev>
Co-Authored-By: Max Brunsfeld <max@zed.dev>
Previously, some of those properties such the font weight, style and color
would be mandatory: when the theme didn't specify them, Zed would use a default
value during deserialization. This meant that those default properties would
unconditionally override the base text style, causing a rendering bug when
combining syntax highlights with diagnostic styles.
This commit fixes that by making `HighlightStyle`s more additive: each property
can be set independently and only the properties that theme specifies get
overridden in the base text style.
