LibJS: Add symbol objects

This commit adds the following classes: SymbolObject, SymbolConstructor,
SymbolPrototype, and Symbol. This commit does not introduce any
new functionality to the Object class, so they cannot be used as
property keys in objects.

Libraries/LibJS/Tests/Symbol.for.js

@@ -0,0 +1,30 @@
+load("test-common.js")
+
+try {
+    const localSym = Symbol("foo");
+    const globalSym = Symbol.for("foo");
+
+    assert(localSym !== globalSym);
+    assert(localSym !== Symbol("foo"));
+    assert(globalSym !== Symbol("foo"));
+    assert(globalSym === Symbol.for("foo"));
+    assert(localSym.toString() === "Symbol(foo)");
+    assert(globalSym.toString() === "Symbol(foo)");
+
+    assert(Symbol.for(1).description === "1");
+    assert(Symbol.for(true).description === "true");
+    assert(Symbol.for({}).description === "[object Object]");
+    assert(Symbol.for().description === "undefined");
+    assert(Symbol.for(null).description === "null");
+
+    assertThrowsError(() => {
+        Symbol.for(Symbol());
+    }, {
+        error: TypeError,
+        message: "Can't convert symbol to string",
+    });
+
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}