fix(ts): change readonly array syntax to support older tsc (#385)

`readonly T[]` is new syntax for `ReadonlyArray` which is supported in typescript older than 3.4.
As of now everything works but throws error while compiling in angular 7

src/util.ts

@@ -471,9 +471,9 @@ export function getType(object: unknown): string {
   return type;
 }
 
-export function copyAndExtendArray<T>(arr: readonly T[], newValue: T): T[];
+export function copyAndExtendArray<T>(arr: ReadonlyArray<T>, newValue: T): T[];
 export function copyAndExtendArray<A, V>(
-  arr: readonly A[],
+  arr: ReadonlyArray<A>,
   newValue: V
 ): (A | V)[];
 /**
@@ -485,7 +485,7 @@ export function copyAndExtendArray<A, V>(
  * @returns A new array with all items from arr and newValue (which is last).
  */
 export function copyAndExtendArray<A, V>(
-  arr: readonly A[],
+  arr: ReadonlyArray<A>,
   newValue: V
 ): (A | V)[] {
   return [...arr, newValue];