LibCore: Add a way to parse a DateTime from a string

DateTime can now be parsed from a string. Implements the same formatters
as strptime: https://linux.die.net/man/3/strptime (Well, some of them at
least).

Userland/Libraries/LibCore/DateTime.h

@@ -35,6 +35,7 @@ public:
     static DateTime create(unsigned year, unsigned month = 1, unsigned day = 0, unsigned hour = 0, unsigned minute = 0, unsigned second = 0);
     static DateTime now();
     static DateTime from_timestamp(time_t);
+    static Optional<DateTime> parse(const String& format, const String& string);
 
     bool operator<(const DateTime& other) const { return m_timestamp < other.m_timestamp; }