gitignore: Ignore detritus from Flutter DevTools

This devtools_options.yaml file seems to get created whenever I
use the Flutter DevTools, the home of the Flutter inspector.

It's pretty vacuous -- just one line reading "extensions:".
Seems like it'd be better for it to not create it.

But in the meantime we can make it not a problem for ourselves,
by just telling Git to ignore it so it doesn't clutter things up.

Author: gnprice

.gitignore

@@ -25,6 +25,7 @@ migrate_working_dir/
 **/doc/api/
 **/ios/Flutter/.last_build_id
 .dart_tool/
+/devtools_options.yaml
 .flutter-plugins
 .flutter-plugins-dependencies
 .packages