Implement text/didChange

[iamrecursion]

Summary

In addition to text/applyEdit (#549), the other key portion of the text editing workflow is the ability for the language server to inform clients about changes. This is both useful for ensuring that multiple clients stay in sync, as well as allowing the language server to perform broader changes to files that the clients have open where necessary.

Value

We can ensure that the clients and the language server stay in sync with regards to their views of the open buffers.

Specification

• Implement the text/didChange message as specified in the design document.
• Ensure that when text/applyEdit is successfully applied, all other clients are informed about it by being sent a text/applyEdit.
• Document any errors that may be associated with this message in the design document.

Acceptance Criteria & Test Cases

• The above specification has been implemented.
• The above functionality has been rigorously tested.

[sylwiabr]

implemented together with #549