LibJS: Handle hex and unicode escape sequences in string literals

Introduces the following syntax:

'\x55'
'\u26a0'
'\u{1f41e}'

Libraries/LibJS/Tests/string-escapes.js

@@ -0,0 +1,17 @@
+load("test-common.js")
+
+try {
+    assert("\x55" === "U");
+    assert("\X55" === "X55");
+    assert(`\x55` === "U");
+    assert(`\X55` === "X55");
+
+    assert("\u26a0" === "⚠");
+    assert(`\u26a0` === "⚠");
+    assert("\u{1f41e}" === "🐞");
+    assert(`\u{1f41e}` === "🐞");
+    
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}