Improve support for language-specific default override indicators

[rzhao271]

Ref #131707

There is currently an issue in the configuration service that prevents us from reliably identifying which extension contributed a language-specific default override.

1. In a sample extension, change its contributes.configurationDefaults such that it contributes a language-specific default override.
2. Run the extension and search for the language-specific setting that is overridden. Remember to use a language filter.
3. The default override indicator does not show up in that view.

Note that the default override indicator is scope-specific. When an extension overrides the default of a language-specific setting, when viewing the non-language-specific equivalent of the setting, my current belief is the default override indicator should not show up.