add generic timestamp

[confusedalex]

This impelements a sealed [1] class which sort of acts as a union tag. This will give us exhaustive switching for every timestamp, like this:

    section.visit<OrgGenericTimestamp>((node) {
      switch (node) { // => error
          // The type 'OrgGenericTimestamp' is not exhaustively matched by
          // the switch cases since it doesn't match OrgTimeRangeTimestamp()'.

          // do something
          break;

        case OrgSimpleTimestamp():
          // do something
          break;
      }
      return true;
    });

Also, because the OrgGenericTimestamp extends OrgNode, we can access toMarkup() and other methods directly from the OrgGenericTimestamp.

[1] https://dart.dev/language/class-modifiers#sealed

[amake]

Thanks for this! I figured there was a good way to do this, but coming from Java I've always found Dart's soup of keywords in this area to be confusing. I suspect there are some other groups of OrgNodes that could use the same treatment.

I thought we might as well call the common interface OrgTimestamp. Let me know if that's OK with you.

[confusedalex]

Defining a interface and consolidating common attributes sound good!