move game state and head interfaces into shared file

src/interfaces.ts

@@ -2,3 +2,27 @@ export interface IPosition {
 	ply: number;
 	fen: string;
 }
+
+export interface IHeader {
+	event: string;
+	site: string;
+	date: string;
+	round: string;
+	white: string;
+	black: string;
+	result: string;
+	eco: string;
+	whiteElo: string;
+	blackElo: string;
+	plyCount: string;
+	eventDate: string;
+	source: string;
+	[key: string]: string;
+}
+
+export interface IGameState {
+	pgn: string;
+	headers: IHeader;
+	diagrams: IPosition[];
+	diagramClock: boolean;
+}