Allow Diagnostic's message to be MarkedString

[octref]

The use case is to show an error's documentation online.

For example, in eslint-plugin-vue, if I'm giving back a Diagnostic of attribute-hyphenation, I'd like to include in the message something like

[attribute-hyphenation](https://github.com/vuejs/eslint-plugin-vue/blob/master/docs/rules/attribute-hyphenation.md)

[rcjsuen]

@octref You should open a bug against the Microsoft/language-server-protocol repository instead if you want to make suggestions to the protocol.

As to your particular problem, please see microsoft/language-server-protocol#250.

[octref]

I see, I didn't find that issue. Thanks for linking.