Emit `MarkupContent` instead of `MarkedString[]` for hover messages

Currently, lsif-java uses `MarkedString[]` to emit hover messages. This format is considered deprecated and we should use `MarkupContent` instead. The benefit of `MarkupContent` is that we should be able to get correct syntax highlighting in the hover messages. Currently, lsif-java hover messages are rendered as monospace with no syntax highlighting.