fix(lib): preserve readonly narrowing in Array.isArray type guard

Adds an overload to ArrayConstructor.isArray that narrows
readonly array types correctly, fixing a 9-year-old issue where
Array.isArray() would lose readonly type information.

Fixes #17002

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: daishuge

src/lib/es5.d.ts

@@ -1499,6 +1499,7 @@ interface ArrayConstructor {
     (arrayLength?: number): any[];
     <T>(arrayLength: number): T[];
     <T>(...items: T[]): T[];
+    isArray(arg: readonly any[] | any): arg is readonly any[];
     isArray(arg: any): arg is any[];
     readonly prototype: any[];
 }

tests/cases/compiler/arrayIsArrayReadonlyNarrowing.ts

@@ -0,0 +1,26 @@
+// @strict: true
+
+function test1(x: readonly string[] | string) {
+    if (Array.isArray(x)) {
+        x; // should be readonly string[]
+    }
+}
+
+function test2(x: readonly number[] | number) {
+    if (Array.isArray(x)) {
+        x; // should be readonly number[]
+        x[0]; // should be number
+    }
+}
+
+function test3(x: string[] | string) {
+    if (Array.isArray(x)) {
+        x; // should still be string[] (not readonly)
+    }
+}
+
+function test4(x: unknown) {
+    if (Array.isArray(x)) {
+        x; // should still be any[] (existing behavior)
+    }
+}