LibJS: Allow Unicode escape sequences in identifiers

For example, "property.br\u{64}wn" should resolve to "property.brown".

To support this behavior, this commit changes the Token class to hold
both the evaluated identifier name and a view into the original source
for the unevaluated name. There are some contexts in which identifiers
are not allowed to contain Unicode escape sequences; for example, export
statements of the form "export {} from foo.js" forbid escapes in the
identifier "from".

The test file is added to .prettierignore because prettier will replace
all escaped Unicode sequences with their unescaped value.

Userland/Libraries/LibJS/Tests/unicode-identifier-escape.js

@@ -0,0 +1,19 @@
+test("basic escapes", () => {
+    var foo = {};
+    foo.brown = 12389;
+
+    expect(foo.brown).toBe(12389);
+    expect(foo.br\u006fwn).toBe(12389);
+    expect(foo.br\u{6f}wn).toBe(12389);
+    expect(foo.\u{62}\u{72}\u{6f}\u{77}\u{6e}).toBe(12389);
+});
+
+test("non-ascii escapes", () => {
+    var foo = {};
+    foo.𝓑𝓻𝓸𝔀𝓷 = 12389;
+
+    expect(foo.𝓑𝓻𝓸𝔀𝓷).toBe(12389);
+    expect(foo.𝓑𝓻\ud835\udcf8𝔀𝓷).toBe(12389);
+    expect(foo.𝓑𝓻\u{1d4f8}𝔀𝓷).toBe(12389);
+    expect(foo.\u{1d4d1}\u{1d4fb}\u{1d4f8}\u{1d500}\u{1d4f7}).toBe(12389);
+});