Implement InlayHint (#19)

Co-authored-by: ykiko <ykikoykikoykiko@gmail.com>

include/Basic/Document.h

@@ -47,6 +47,24 @@ struct TextDocumentPositionParams {
     Position position;
 };
 
+
+struct MarkupKind {
+    std::string_view m_value;
+
+    constexpr MarkupKind(std::string_view value) : m_value(value) {}
+
+    constexpr static std::string_view PlainText = "plaintext";
+    constexpr static std::string_view Markdown = "markdown";
+};
+
+struct MarkupContent {
+    /// The type of the Markup.
+    MarkupKind kind;
+
+    /// The content itself.
+    string value;
+ };
+
 struct DidOpenTextDocumentParams {
     /// The document that was opened.
     TextDocumentItem textDocument;