LibJS: Add all of the FinalizationRegistry.prototype methods

More specifically: cleanupSome, register & unregister.

FinalizationRegistery.prototype.cleanupSome is actually still a stage 2
proposal, but since test262 test cases already exist for it, i decided
to go for it :)

Userland/Libraries/LibJS/Tests/builtins/FinalizationRegistry/FinalizationRegistry.prototype.cleanupSome.js

@@ -0,0 +1,35 @@
+test("length is 0", () => {
+    expect(FinalizationRegistry.prototype.cleanupSome).toHaveLength(0);
+});
+
+function registerInDifferentScope(registry) {
+    registry.register({}, {});
+}
+
+test("basic functionality", () => {
+    var registry = new FinalizationRegistry(() => {});
+
+    var count = 0;
+    var increment = () => {
+        count++;
+    };
+
+    registry.cleanupSome(increment);
+
+    expect(count).toBe(0);
+
+    registerInDifferentScope(registry);
+    gc();
+
+    registry.cleanupSome(increment);
+
+    expect(count).toBe(1);
+});
+
+test("errors", () => {
+    var registry = new FinalizationRegistry(() => {});
+
+    expect(() => {
+        registry.cleanupSome(5);
+    }).toThrowWithMessage(TypeError, "is not a function");
+});

Userland/Libraries/LibJS/Tests/builtins/FinalizationRegistry/FinalizationRegistry.prototype.register.js

@@ -0,0 +1,35 @@
+test("length is 2", () => {
+    expect(FinalizationRegistry.prototype.register).toHaveLength(2);
+});
+
+test("basic functionality", () => {
+    var registry = new FinalizationRegistry(() => {});
+
+    var target1 = {};
+    var heldValue1 = {};
+
+    registry.register(target1, heldValue1);
+
+    var target2 = {};
+    var heldValue2 = {};
+    var token = {};
+
+    registry.register(target2, heldValue2, token);
+});
+
+test("errors", () => {
+    var registry = new FinalizationRegistry(() => {});
+
+    expect(() => {
+        registry.register(5, {});
+    }).toThrowWithMessage(TypeError, "is not an object");
+
+    expect(() => {
+        var a = {};
+        registry.register(a, a);
+    }).toThrowWithMessage(TypeError, "Target and held value must not be the same");
+
+    expect(() => {
+        registry.register({}, {}, 5);
+    }).toThrowWithMessage(TypeError, "is not an object");
+});

Userland/Libraries/LibJS/Tests/builtins/FinalizationRegistry/FinalizationRegistry.prototype.unregister.js

@@ -0,0 +1,25 @@
+test("length is 2", () => {
+    expect(FinalizationRegistry.prototype.unregister).toHaveLength(1);
+});
+
+test("basic functionality", () => {
+    var registry = new FinalizationRegistry(() => {});
+
+    var target = {};
+    var heldValue = {};
+    var token = {};
+
+    registry.register(target, heldValue, token);
+
+    expect(registry.unregister({})).toBe(false);
+    expect(registry.unregister(token)).toBe(true);
+    expect(registry.unregister(token)).toBe(false);
+});
+
+test("errors", () => {
+    var registry = new FinalizationRegistry(() => {});
+
+    expect(() => {
+        registry.unregister(5);
+    }).toThrowWithMessage(TypeError, "is not an object");
+});