Currently once you retrieve the lenses you're pretty much stuck with
them as saving new lenses is additive.

Adding a dedicated method to reset lenses allows users to toggle lenses
on/off which can be useful for language servers where they are noisy or
expensive and you only want to see them temporary.

Also adds a check against the server capabilities to fix
https://github.com/neovim/neovim/issues/15183

Previously, the handler signature was:

  function(err, method, params, client_id, bufnr, config)

In order to better support external plugins that wish to extend the
protocol, there is other information which would be advantageous to
forward to the client, such as the original params of the request that
generated the callback.

In order to do this, we would need to break symmetry of the handlers, to
add an additional "params" as the 7th argument.

Instead, this PR changes the signature of the handlers to:

  function(err, result, ctx, config)

where ctx (the context) includes params, client_id, and bufnr. This also leaves
flexibility for future use-cases.

BREAKING_CHANGE: changes the signature of the built-in client handlers, requiring
updating handler calls