Update DLS to newer-style LSP config update scheme

[JonatanWaern]

Prior to LSP 3.6, seemingly config was updated in a client->server push manner.

After this, the correct way to do this became more convoluted.

The DLS needs to:

• Check client for dynamic registration capability on 'DidChangeConfigurationClientCapabilitites'
• If true, register for it using a 'registerCapability' request
• Then, the server will receive an 'empty' (likely, settings: undefined or empty array) notification when the client updates any configuration
• As a response to this, the server should send a 'ConfigurationRequest' to the client with all relevant config settings
• And then use the response from the client to update its internal settings

We might in addition to this want to also keep support for old-style push config changes

[JonatanWaern]

Resolved in #105