LibJS: Add initial implementation for SharedArrayBuffer

None of the actual sharing is implemented yet, but this is enough for
most basic functionality.

Diff Tests:
    +260 ✅    -262 ❌   +2 💀

Userland/Libraries/LibJS/Tests/builtins/SharedArrayBuffer/SharedArrayBuffer.js

@@ -0,0 +1,19 @@
+test("basic functionality", () => {
+    expect(SharedArrayBuffer).toHaveLength(1);
+    expect(SharedArrayBuffer.name).toBe("SharedArrayBuffer");
+    expect(SharedArrayBuffer.prototype.constructor).toBe(SharedArrayBuffer);
+    expect(new SharedArrayBuffer()).toBeInstanceOf(SharedArrayBuffer);
+    expect(typeof new SharedArrayBuffer()).toBe("object");
+});
+
+test("SharedArrayBuffer constructor must be invoked with 'new'", () => {
+    expect(() => {
+        SharedArrayBuffer();
+    }).toThrowWithMessage(TypeError, "SharedArrayBuffer constructor must be called with 'new'");
+});
+
+test("SharedArrayBuffer size limit", () => {
+    expect(() => {
+        new SharedArrayBuffer(2 ** 53);
+    }).toThrowWithMessage(RangeError, "Invalid shared array buffer length");
+});

Userland/Libraries/LibJS/Tests/builtins/SharedArrayBuffer/SharedArrayBuffer.prototype.byteLength.js

@@ -0,0 +1,6 @@
+test("basic functionality", () => {
+    expect(new SharedArrayBuffer().byteLength).toBe(0);
+    expect(new SharedArrayBuffer(1).byteLength).toBe(1);
+    expect(new SharedArrayBuffer(64).byteLength).toBe(64);
+    expect(new SharedArrayBuffer(123).byteLength).toBe(123);
+});

Userland/Libraries/LibJS/Tests/builtins/SharedArrayBuffer/SharedArrayBuffer.prototype.slice.js

@@ -0,0 +1,29 @@
+test("single parameter", () => {
+    const buffer = new SharedArrayBuffer(16);
+    const fullView = new Int32Array(buffer);
+
+    // modify some value that we can check in the sliced buffer
+    fullView[3] = 7;
+
+    // slice the buffer and use a new int32 view to perform basic checks
+    const slicedBuffer = buffer.slice(12);
+    const slicedView = new Int32Array(slicedBuffer);
+
+    expect(slicedView).toHaveLength(1);
+    expect(slicedView[0]).toBe(7);
+});
+
+test("both parameters", () => {
+    const buffer = new SharedArrayBuffer(16);
+    const fullView = new Int32Array(buffer);
+
+    // modify some value that we can check in the sliced buffer
+    fullView[1] = 12;
+
+    // slice the buffer and use a new int32 view to perform basic checks
+    const slicedBuffer = buffer.slice(4, 8);
+    const slicedView = new Int32Array(slicedBuffer);
+
+    expect(slicedView).toHaveLength(1);
+    expect(slicedView[0]).toBe(12);
+});