x/tools/gopls: specialize file watching with a setting rather than user agent

[findleyr]

          Thinking more, I am more convinced that gopls shouldn't use the `ClientName` in adjusting its functionality.
          For example, we should've used an explicit setting instead of using the ClientName in  https://github.com/golang/tools/blob/f1f7c26696be8f94198beafd05b36843515ac2ba/gopls/internal/cache/snapshot.go#L950

Originally posted by @hyangah in #70205 (comment)

Extracting this to an issue: for some reason (expedience, most likely) I chose to specialize file watching patterns based on the VS Code user agent (because VS Code performs very poorly with certain watch patterns). A more explicit (and therefore better) solution would have been to make the VS Code Go client inject a setting that selects the most appropriate watch patterns.