fix(classes,packages): Use ex-height font adjustment for code

Rather than a hard-coded 3pt font size adjustment that
cannot be general with any font.

Impacts the code command from the plain class, and also the urlstyle
default implementation from the url package as it relies on the latter.

Author: Omikhleia

classes/plain.lua

@@ -262,7 +262,9 @@ function class:registerCommands ()
       -- definition from the former, but it's of sub-quality...
       -- The ugly -3 size is a HACK of sorts.
       options.family = options.family or "Hack"
-      options.size = options.size or SILE.settings:get("font.size") - 3
+      if not options.size and not options.adjust then
+         options.adjust = "ex-height"
+      end
       SILE.call("font", options, content)
    end)