LibJS: Make (most) String.prototype functions generic

I.e. they don't require the |this| value to be a string object and
"can be transferred to other kinds of objects for use as a method" as
the spec describes it.

Libraries/LibJS/Tests/String.prototype-generic-functions.js

@@ -0,0 +1,51 @@
+load("test-common.js");
+
+try {
+    const genericStringPrototypeFunctions = [
+        "charAt",
+        "repeat",
+        "startsWith",
+        "indexOf",
+        "toLowerCase",
+        "toUpperCase",
+        "padStart",
+        "padEnd",
+        "trim",
+        "trimStart",
+        "trimEnd",
+        "concat",
+        "substring",
+        "includes",
+    ];
+
+    genericStringPrototypeFunctions.forEach(name => {
+        String.prototype[name].call({ toString: () => "hello friends" });
+        String.prototype[name].call({ toString: () => 123 });
+        String.prototype[name].call({ toString: () => undefined });
+
+        assertThrowsError(() => {
+            String.prototype[name].call({ toString: () => new String() });
+        }, {
+            error: TypeError,
+            message: "Cannot convert object to string"
+        });
+
+        assertThrowsError(() => {
+            String.prototype[name].call({ toString: () => [] });
+        }, {
+            error: TypeError,
+            message: "Cannot convert object to string"
+        });
+
+        assertThrowsError(() => {
+            String.prototype[name].call({ toString: () => ({}) });
+        }, {
+            error: TypeError,
+            message: "Cannot convert object to string"
+        });
+    });
+
+    console.log("PASS");
+} catch (err) {
+    console.log("FAIL: " + err);
+}