Allow service plugins to define request / notification handlers.

[remcohaszing]

Based on mdx-js/mdx-analyzer#384, a bit of boilerplate is needed to define custom LSP request and notification handlers.

I think it would be neat if service plugins can specify request and notification handlers. This means language servers no longer have to deal with any of this.

The following service plugin:

export function createMdxServicePlugin() {
  return {
    name: 'mdx',

    create(context) {
      return {
        notification: {
          sayHi(params) {
            // Handle notification
          }
        },

        request: {
          toggleStrong(params) {
            // Handle request
          }
        }
      }
    }
  }
}

Would be equivalent to the following language server code:

connection.onRequest('mdx/toggleStrong', params => {
  // Handle request
})

connection.onNotification('mdx/sayHi', params => {
  // Handle request
})

The service plugin name is used to scope the requests and notifications, and is therefor required.

This could also make usage from contexts other than a language server more straight-forward, e.g. for testing purposes or usage from Monaco editor.