fix: Fix wrong param type in definitions for String.fromCharCodes (#2316)

Author: MaxGraey

std/assembly/index.d.ts

@@ -1802,7 +1802,7 @@ declare class StaticArray<T> {
 /** Class representing a sequence of characters. */
 declare class String {
   static fromCharCode(ls: i32, hs?: i32): string;
-  static fromCharCodes(arr: u16[]): string;
+  static fromCharCodes(arr: i32[]): string;
   static fromCodePoint(code: i32): string;
   static fromCodePoints(arr: i32[]): string;
   static raw(parts: TemplateStringsArray, ...args: any[]): string;

std/portable/index.d.ts

@@ -309,7 +309,7 @@ declare const JSMath: typeof Math;
 
 declare interface StringConstructor {
   /** Equivalent to calling `String.fromCharCode` with multiple arguments. */
-  fromCharCodes(arr: u16[]): string;
+  fromCharCodes(arr: i32[]): string;
   /** Equivalent to calling `String.fromCodePoint` with multiple arguments. */
   fromCodePoints(arr: i32[]): string;
 }