Pass rc.weekstart to libshared for ISO8601 weeknum parsing if "monday" (#3654)

* libshared: bump for weekstart, epoch defines, eopww fix

mainly those visible changes, and miscellaneous others

see GothenburgBitFactory/taskwarrior#3623 (weekstart)
see GothenburgBitFactory/taskwarrior#3651 (epoch limit defines)
see GothenburgBitFactory/libshared#73 (eopww fix)

* Initialize libshared's weekstart from user's rc.weekstart config

This enables use of newer libshared code that can parse week numbers
according to ISO8601 instead of existing code which is always using
Sunday-based weeks.  To get ISO behavior, set rc.weekstart=monday.
Default is still Sunday / old algorithm, as before, since Sunday is in
the hardcoded default rcfile.

Weekstart does not yet fix week-relative shortcuts, which will still
always use Monday.

See #3623 for further details.

src/Context.cpp

@@ -1105,6 +1105,11 @@ void Context::staticInitialization() {
   Task::regex = Variant::searchUsingRegex = config.getBoolean("regex");
   Lexer::dateFormat = Variant::dateFormat = config.get("dateformat");
 
+  auto weekStart = Datetime::dayOfWeek(config.get("weekstart"));
+  if (weekStart != 0 && weekStart != 1)
+    throw std::string(
+        "The 'weekstart' configuration variable may only contain 'Sunday' or 'Monday'.");
+  Datetime::weekstart = weekStart;
   Datetime::isoEnabled = config.getBoolean("date.iso");
   Datetime::standaloneDateEnabled = false;
   Datetime::standaloneTimeEnabled = false;