LibUnicode: Parse and generate relative-time format patterns

Relative-time format patterns are of one of two forms:

    * Tensed - refer to the past or the future, e.g. "N years ago" or
      "in N years".
    * Numbered - refer to a specific numeric value, e.g. "in 1 year"
      becomes "next year" and "in 0 years" becomes "this year".

In ECMA-402, tensed and numbered refer to the numeric formatting options
of "always" and "auto", respectively.

Userland/Libraries/LibUnicode/RelativeTimeFormat.h

@@ -0,0 +1,48 @@
+/*
+ * Copyright (c) 2022, Tim Flynn <trflynn89@pm.me>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Optional.h>
+#include <AK/StringView.h>
+#include <AK/Vector.h>
+#include <LibUnicode/Forward.h>
+#include <LibUnicode/Locale.h>
+
+namespace Unicode {
+
+// These are just the subset of fields in the CLDR required for ECMA-402.
+enum class TimeUnit {
+    Second,
+    Minute,
+    Hour,
+    Day,
+    Week,
+    Month,
+    Quarter,
+    Year,
+};
+
+struct RelativeTimeFormat {
+    enum class Plurality {
+        Zero,
+        One,
+        Two,
+        Few,
+        Many,
+        Other,
+    };
+
+    Plurality plurality { Plurality::Other };
+    StringView pattern;
+};
+
+Optional<TimeUnit> time_unit_from_string(StringView time_unit);
+StringView time_unit_to_string(TimeUnit time_unit);
+
+Vector<RelativeTimeFormat> get_relative_time_format_patterns(StringView locale, TimeUnit time_unit, StringView tense_or_number, Style style);
+
+}