Convert to use MarkedString

[octref]

MarkedString is deprecated and causes confusion. See microsoft/vscode#62332

[aeschli]

Implementation note:
MarkedString might not be supported by every client of the service. To let clients specify what capabilities they have, I started in the JSON-languageservice that the client can pass in a ClientCapabilty to the service: See https://github.com/Microsoft/vscode-json-languageservice/blob/7b0bc0dcf45ab6411429e986716f969f92773850/src/jsonLanguageTypes.ts#L207

[octref]

Will do this together with HTML.

Also, microsoft/vscode-html-languageservice#12 needs to be handled as well. Per the policy we need to show warning when linking to non-Microsoft websites.