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/CMakeLists.txt

@@ -13,6 +13,7 @@ set(SOURCES
     DateTimeFormat.cpp
     Locale.cpp
     NumberFormat.cpp
+    RelativeTimeFormat.cpp
 )
 
 serenity_lib(LibUnicode unicode)