Change deprecated MarkedString to MarkdownString

[BugDiver]

Language server protocol has deprecated the MarkedString and they are replacing it with MarkdownString.
Please update the lsp library.
Ref :-

https://github.com/Microsoft/language-server-protocol/blob/master/protocol.md#new-markupcontent

https://github.com/Microsoft/vscode/blob/d8989c95964ce06c64ffebc2fb400a0327bd447d/src/vs/vscode.d.ts#L2012

[keegancsmith]

I'll happily accept a PR along these lines.

[BugDiver]

@keegancsmith I have created a PR for it. Please have a look and provide your valuable feedback.