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");
+});