chore: fix types of component interface (#7097)

Author: sscaff1

packages/decap-cms-core/index.d.ts

@@ -426,12 +426,12 @@ declare module 'decap-cms-core' {
   export interface EditorComponentOptions {
     id: string;
     label: string;
-    fields: EditorComponentField[];
+    fields?: EditorComponentField[];
     pattern: RegExp;
     allow_add?: boolean;
     fromBlock: (match: RegExpMatchArray) => any;
     toBlock: (data: any) => string;
-    toPreview: (data: any) => string;
+    toPreview: (data: any) => string | JSX.Element;
   }
 
   export interface PreviewStyleOptions {