You can now (VSCode 1.68, May 2022) do this through the Settings GUI instead of directly your settings.json
.
The Settings editor now shows a default value override indicator for language-specific settings.
As a note, one can view language-specific settings by adding a language filter in the Settings editor search bar, and one can add such a filter either by typing it out explicitly, or by clicking the filter button on the right of the search bar, and selecting the "Language" option.
When the default value override indicator shows up, it indicates that the default value of the language-specific setting has been overridden by an extension. The indicator also indicates which extension overrode the default value.

(Theme Light Pink)
This example above is for line wrapping, but you can adapt it to reference trim_trailing_whitespace
.