LibUnicode: Implement grammar validators for Unicode TR-35

ECMA-402 requires validating user input against the EBNF grammar for
Unicode locales described in TR-35: https://www.unicode.org/reports/tr35

This commit adds validators for that grammar, as well as other helper to
e.g. canonicalize a locale string.

Userland/Libraries/LibUnicode/Locale.h

@@ -0,0 +1,40 @@
+/*
+ * Copyright (c) 2021, Tim Flynn <trflynn89@pm.me>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Optional.h>
+#include <AK/String.h>
+#include <AK/StringView.h>
+#include <AK/Vector.h>
+#include <LibUnicode/Forward.h>
+
+namespace Unicode {
+
+struct LanguageID {
+    bool is_root { false };
+    Optional<StringView> language {};
+    Optional<StringView> script {};
+    Optional<StringView> region {};
+    Vector<StringView> variants {};
+};
+
+struct LocaleID {
+    LanguageID language_id {};
+};
+
+// Note: These methods only verify that the provided strings match the EBNF grammar of the
+// Unicode identifier subtag (i.e. no validation is done that the tags actually exist).
+bool is_unicode_language_subtag(StringView);
+bool is_unicode_script_subtag(StringView);
+bool is_unicode_region_subtag(StringView);
+bool is_unicode_variant_subtag(StringView);
+
+Optional<LanguageID> parse_unicode_language_id(StringView);
+Optional<LocaleID> parse_unicode_locale_id(StringView);
+Optional<String> canonicalize_unicode_locale_id(LocaleID&);
+
+}