Fix Lint errors in StringUtility.ts (#479)

src/Shared/StorageUtility.ts

@@ -1,4 +1,4 @@
-import { StringUtility } from "./StringUtility";
+import * as StringUtility from "./StringUtility";
 
 export class LocalStorageUtility {
   public static hasItem(key: StorageKey): boolean {

src/Shared/StringUtility.test.ts

@@ -1,4 +1,4 @@
-import { StringUtility } from "./StringUtility";
+import * as StringUtility from "./StringUtility";
 
 describe("String utility", () => {
   it("Convert to integer from string", () => {

src/Shared/StringUtility.ts

@@ -1,9 +1,7 @@
-export class StringUtility {
-  public static toNumber(num: string | null): number {
-    return Number(num);
-  }
-
-  public static toBoolean(valueStr: string | null): boolean {
-    return valueStr === "true";
-  }
+export function toNumber(num: string | null): number {
+  return Number(num);
+}
+
+export function toBoolean(valueStr: string | null): boolean {
+  return valueStr === "true";
 }