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

src/file/paragraph/math/n-ary/math-base.ts

@@ -4,7 +4,7 @@ import { XmlComponent } from "@file/xml-components";
 import { MathComponent } from "../math-component";
 
 export class MathBase extends XmlComponent {
-    public constructor(children: MathComponent[]) {
+    public constructor(children: readonly MathComponent[]) {
         super("m:e");
 
         for (const child of children) {

src/file/paragraph/math/n-ary/math-sub-script.ts

@@ -4,7 +4,7 @@ import { XmlComponent } from "@file/xml-components";
 import { MathComponent } from "../math-component";
 
 export class MathSubScriptElement extends XmlComponent {
-    public constructor(children: MathComponent[]) {
+    public constructor(children: readonly MathComponent[]) {
         super("m:sub");
 
         for (const child of children) {

src/file/paragraph/math/n-ary/math-sum.ts

@@ -8,9 +8,9 @@ import { MathSubScriptElement } from "./math-sub-script";
 import { MathSuperScriptElement } from "./math-super-script";
 
 export interface IMathSumOptions {
-    readonly children: MathComponent[];
-    readonly subScript?: MathComponent[];
-    readonly superScript?: MathComponent[];
+    readonly children: readonly MathComponent[];
+    readonly subScript?: readonly MathComponent[];
+    readonly superScript?: readonly MathComponent[];
 }
 
 export class MathSum extends XmlComponent {

src/file/paragraph/math/n-ary/math-super-script.ts

@@ -4,7 +4,7 @@ import { XmlComponent } from "@file/xml-components";
 import { MathComponent } from "../math-component";
 
 export class MathSuperScriptElement extends XmlComponent {
-    public constructor(children: MathComponent[]) {
+    public constructor(children: readonly MathComponent[]) {
         super("m:sup");
 
         for (const child of children) {