LibJS: Implement tagged template literals (foobar)

To make processing tagged template literals easier, template literals
will now add one empty StringLiteral before and after each template
expression *if* there's no other string - e.g.:

`${foo}` -> "", foo, ""
`test${foo}${bar}test` -> "test", foo, "", bar, "test"

This also matches the behaviour of many other parsers.

Libraries/LibJS/Tests/tagged-template-literals.js

@@ -0,0 +1,88 @@
+load("test-common.js");
+
+try {
+    assertThrowsError(() => {
+        foo`bar${baz}`;
+    }, {
+        error: ReferenceError,
+        message: "'foo' not known"
+    });
+
+    assertThrowsError(() => {
+        function foo() { }
+        foo`bar${baz}`;
+    }, {
+        error: ReferenceError,
+        message: "'baz' not known"
+    });
+
+    assertThrowsError(() => {
+        undefined``````;
+    }, {
+        error: TypeError,
+        message: "undefined is not a function"
+    });
+
+    function test1(strings) {
+        assert(strings instanceof Array);
+        assert(strings.length === 1);
+        assert(strings[0] === "");
+        return 42;
+    }
+    assert(test1`` === 42);
+
+    function test2(s) {
+        return function (strings) {
+            assert(strings instanceof Array);
+            assert(strings.length === 1);
+            assert(strings[0] === "bar");
+            return s + strings[0];
+        }
+    }
+    assert(test2("foo")`bar` === "foobar");
+
+    var test3 = {
+        foo(strings, p1) {
+            assert(strings instanceof Array);
+            assert(strings.length === 2);
+            assert(strings[0] === "");
+            assert(strings[1] === "");
+            assert(p1 === "bar");
+        }
+    };
+    test3.foo`${"bar"}`;
+
+    function test4(strings, p1) {
+        assert(strings instanceof Array);
+        assert(strings.length === 2);
+        assert(strings[0] === "foo");
+        assert(strings[1] === "");
+        assert(p1 === 42);
+    }
+    var bar = 42;
+    test4`foo${bar}`;
+
+    function test5(strings, p1, p2) {
+        assert(strings instanceof Array);
+        assert(strings.length === 3);
+        assert(strings[0] === "foo");
+        assert(strings[1] === "baz");
+        assert(strings[2] === "");
+        assert(p1 === 42);
+        assert(p2 === "qux");
+        return (strings, value) => `${value}${strings[0]}`;
+    }
+    var bar = 42;
+    assert(test5`foo${bar}baz${"qux"}``test${123}` === "123test");
+
+    function review(strings, name, rating) {
+        return `${strings[0]}**${name}**${strings[1]}_${rating}_${strings[2]}`;
+    }
+    var name = "SerenityOS";
+    var rating = "great";
+    assert(review`${name} is a ${rating} project!` === "**SerenityOS** is a _great_ project!");
+
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}