x/tools/gopls: add a mechanism for marking a setting deprecated in documentation

[stamblerre]

We currently have a way of notifying users that they are using a deprecated setting, but it would also be nice to mark the deprecation in the documentation. This way, we could surface the deprecation in the package.json and smoothly transition users off of an outdated setting.

/cc @heschik @hyangah

Edit: This would be doable by adding a struct tag that sets a field's status to "deprecated". We could then use reflection to check the field's status when setting it. However, we should probably only do this once we set all options using reflection, which is a big enough change that we shouldn't do it until after February.

[findleyr]

This is not urgent, but should be straightforward and would be nice to do before we move configuration around. Tentatively putting in the v0.8.0 milestone.