LibCore+LibTimeZone: Support time zone names in Core::DateTime::parse

LibCore currently cannot depend on LibTimeZone directly. All build-time
code generators depend on LibCore, so there'd be a circular dependency:
LibCore -> LibTimeZone -> GenerateTZData -> LibCore.

So to support parsing time zone names and applying their offsets, add a
couple of weakly-defined helper functions. These work similar to the way
AK::String declares some methods that LibUnicode defines. Any user who
wants to parse time zone names (from outside of LibCore itself) can link
against LibTimeZone to receive full support.

Userland/Libraries/LibTimeZone/DateTime.cpp

@@ -0,0 +1,38 @@
+/*
+ * Copyright (c) 2023, Tim Flynn <trflynn89@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/GenericLexer.h>
+#include <LibTimeZone/DateTime.h>
+#include <LibTimeZone/TimeZone.h>
+
+namespace Core {
+
+Optional<StringView> parse_time_zone_name(GenericLexer& lexer)
+{
+    auto start_position = lexer.tell();
+
+    Optional<StringView> canonicalized_time_zone;
+
+    lexer.ignore_until([&](auto) {
+        auto time_zone = lexer.input().substring_view(start_position, lexer.tell() - start_position + 1);
+
+        canonicalized_time_zone = TimeZone::canonicalize_time_zone(time_zone);
+        return canonicalized_time_zone.has_value();
+    });
+
+    if (canonicalized_time_zone.has_value())
+        lexer.ignore();
+
+    return canonicalized_time_zone;
+}
+
+void apply_time_zone_offset(StringView time_zone, UnixDateTime& time)
+{
+    if (auto offset = TimeZone::get_time_zone_offset(time_zone, time); offset.has_value())
+        time -= Duration::from_seconds(offset->seconds);
+}
+
+}