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.
* Since regular editors' font sizes and families are controlled by
the settings and not the theme, don't store a dummy text style in
the theme. Instead, only store a font color, and synthesize
the text style for regular editors using both the theme and the
settings.
* Style single-line and auto-height editors (now called "field
editors") using a single function that takes the entire theme and
selects a relevant sub-object.
Co-Authored-By: Nathan Sobo <nathan@zed.dev>
