Use new eslint-plugin-functional instead of tslint-immutable

src/file/document/body/body.ts

@@ -4,6 +4,7 @@ import { IContext, IXmlableObject, XmlComponent } from "@file/xml-components";
 import { ISectionPropertiesOptions, SectionProperties } from "./section-properties/section-properties";
 
 export class Body extends XmlComponent {
+    // eslint-disable-next-line functional/prefer-readonly-type
     private readonly sections: SectionProperties[] = [];
 
     public constructor() {
@@ -16,6 +17,7 @@ export class Body extends XmlComponent {
      * The spec says:
      *  - section element should be in the last paragraph of the section
      *  - last section should be direct child of body
+     *
      * @param options new section options
      */
     public addSection(options: ISectionPropertiesOptions): void {

src/file/document/body/section-properties/properties/columns.ts

@@ -17,7 +17,7 @@ export interface IColumnsAttributes {
     readonly count?: number;
     readonly separate?: boolean;
     readonly equalWidth?: boolean;
-    readonly children?: Column[];
+    readonly children?: readonly Column[];
 }
 
 export class ColumnsAttributes extends XmlAttributeComponent<IColumnsAttributes> {