Allow creating Styles and assigning them to File.Styles

Closes #109

src/file/file.ts

@@ -17,7 +17,7 @@ import { Table } from "./table";
 
 export class File {
     private readonly document: Document;
-    private readonly styles: Styles;
+    private styles: Styles;
     private readonly coreProperties: CoreProperties;
     private readonly numbering: Numbering;
     private readonly media: Media;
@@ -214,6 +214,10 @@ export class File {
         return this.styles;
     }
 
+    public set Styles(styles: Styles) {
+        this.styles = styles;
+    }
+
     public get CoreProperties(): CoreProperties {
         return this.coreProperties;
     }