Try harder to succeed at fall-backing to a Monospace font

A combination of some ideas:

 * Try all Monospace fonts before giving up.
 * Relax exact weight restriction on font matching when trying Monospace
   fall-back. Try smaller weights if needed.
 * Make the fall-back try order weight-offset aware, AND script-aware.
 * And finally, add the option to adjust the font size of glyphs using
   fall-back Monospace fonts, so the width of them matches the default
   font width.

   For my use-case, the current fall-back attempt always fails with
   Arabic script. And none of the Arabic-supporting Monospace fonts in
   my system also support medium weight. So, if my default font is set
   to medium weight, script-aware fall-back alone will still not work.

Signed-off-by: Mohammad AlSaleh <CE.Mohammad.AlSaleh@gmail.com>

Cargo.toml

@@ -24,6 +24,7 @@ self_cell = "1.0.1"
 swash = { version = "0.1.8", optional = true }
 syntect = { version = "5.1.0", optional = true }
 sys-locale = { version = "0.3.1", optional = true }
+ttf-parser = "0.20.0"
 unicode-linebreak = "0.1.5"
 unicode-script = "0.5.5"
 unicode-segmentation = "1.10.1"