Simplify code in `search-filter.js` in preparation for enabling
typechecking of `src/sidebar/services`.

 - Use a single regex to parse the `<time><unit>` expression rather than
   two expressions per time unit.

 - Replace the user of manual offsets to get the field value which is
   error prone.

 - Update JSDoc of various functions

 - Provide a fallback in case a tokenizing regex fails to match. I
   haven't verified that this fallback ever gets used, but it keeps both
   TS and the human reader happier