This sets the stage for dynamically adjusting the sizes of fallback
fonts based on the primary font's face metrics. It also removes a lot of
unnecessary work when loading fallback fonts, since we only actually use
the metrics based on the parimary font.

in preparation to move ownership of metrics from faces to collections

This is an attempt to use `void` as type for Freetype Load Flags when
backend does not use these flags.

This continues our work to improve the amount of explicit error sets
we use in the codebase. Explicit error sets make it easier to understand
possible failure scenarios, allow us to use exhaustive matching, create
compiler errors if errors are unexpectedly added or removed, etc.

The goal eventually is 100% coverage but we're not even close yet.
This just moves us a little closer.

Fixes #2364

This adds the bold, italic, and bold italic variants of JB Mono so it is
built-in. This also fixes up the naming convention for the embedded font
files across tests and removes redundant embedded font files.

This adds a new configuration "font-synthetic-style" to enable or
disable synthetic styles. This is different from "font-style-*" which
specifies a named style or disables a style completely.

Instead, "font-synthetic-style" will disable only the creation of
synthetic styles in the case a font does not support a given style.
This is useful for users who want to obviously know when a font doesn't
support a given style or a user who wants to explicitly only use the
styles that were designed by the font designer.

The default value is to enable all synthetic styles.