refactored out xml components into multiple files

ts/docx/xml-components/unit.ts

@@ -0,0 +1,16 @@
+import {BaseXmlComponent} from "./base";
+
+export abstract class XmlUnitComponent extends BaseXmlComponent {
+    protected root: string;
+
+    constructor(rootKey: string) {
+        super(rootKey);
+    }
+
+    replaceKey(): void {
+        if (this.root !== undefined) {
+            this[this.rootKey] = this.root;
+            delete this.root;
+        }
+    }
+}