Improve CodeDescription or add the ability to provide text documents or handle URIs

[RobbyCBennett]

The CodeDescription type is a dead end if the text isn't a website or file. When this server-defined URI is produced, one would assume that the server could resolve the URI as well. However, this is unfortunately an implementation detail in the clients. For example, VS Code has UriHandler and TextDocumentContentProvider.

Different ideas for improvement:

1. Allow servers to provide CodeDescription text
   • Add a client-to-server request codeDescription/resolve that is given a URI responds with something like a TextDocumentItem
   • Add a field codeDescriptionProvider to ServerCapabilities
2. Allow servers to provide text documents
   • Add a client-to-server request textDocument/resolve that responds with something like a TextDocumentItem
   • Add a field textDocumentProvider to ServerCapabilities
3. Allow servers to perform arbitrary actions with a given URI
   • Add a client-to-server notification workspace/openUri that can do anything such as open a graphical application to display the content of a URI
   • Add a field uriProvider to ServerCapabilities

[RobbyCBennett]

This is similar to #1461 and #1458, but here I give more suggestions.