api: Flatten and simplify Servo preferences (#34966)

Flatten and simplify Servo's preferences code. In addition, have both
preferences and options passed in as arguments to `Servo::new()` and
make sure not to use the globally set preferences in `servoshell` (as
much as possible now).

Instead of a complex procedural macro to generate preferences, just
expose a very simple derive macro that adds string based getters and
setters.

- All command-line parsing is moved to servoshell.
- There is no longer the concept of a missing preference.
- Preferences no longer have to be part of the resources bundle because
  they now have reasonable default values.
- servoshell specific preferences are no longer part of the preferences
  exposed by the Servo API.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/net/decoder.rs

@@ -280,7 +280,7 @@ impl Stream for BodyStream {
                     .content_length
                     .map_or(false, |c| c.0 == self.total_read);
                 if self.is_secure_scheme &&
-                    (all_content_read || pref!(network.tls.ignore_unexpected_eof))
+                    (all_content_read || pref!(network_tls_ignore_unexpected_eof))
                 {
                     let source = err.source();
                     let is_unexpected_eof = source