Improve import alias

@file/ and @export/ instead of file/ and export/ etc

src/file/header/header-attributes.ts

@@ -1,4 +1,4 @@
-import { XmlAttributeComponent } from "file/xml-components";
+import { XmlAttributeComponent } from "@file/xml-components";
 
 export class HeaderAttributes extends XmlAttributeComponent<{
     readonly wpc?: string;

src/file/header/header.spec.ts

@@ -1,6 +1,6 @@
 import { expect } from "chai";
 
-import { Formatter } from "export/formatter";
+import { Formatter } from "@export/formatter";
 
 import { Paragraph } from "../paragraph";
 import { Header } from "./header";

src/file/header/header.ts

@@ -1,5 +1,5 @@
 // http://officeopenxml.com/WPheaders.php
-import { InitializableXmlComponent, XmlComponent } from "file/xml-components";
+import { InitializableXmlComponent, XmlComponent } from "@file/xml-components";
 import { Paragraph } from "../paragraph";
 import { Table } from "../table";
 import { HeaderAttributes } from "./header-attributes";