Fix some clear methods

[pingbird]

The clearScreenAfter / clearScreenBefore and clearLineAfter / clearLineBefore methods were swapped, this PR fixes their order.

A simple program showing the issue:

import 'dart:io';

import 'package:neoansi/neoansi.dart';

void main(List<String> args) async {
  final console = AnsiWriter.from(stdout);

  console.write('#########\n' * 3);
  console.moveCursorUp(2);
  console.moveCursorRight(4);
  if (args.single == 'clearLineAfter') {
    console.clearLineAfter();
  } else if (args.single == 'clearLineBefore') {
    console.clearLineBefore();
  } else if (args.single == 'clearScreenAfter') {
    console.clearScreenAfter();
  } else if (args.single == 'clearScreenBefore') {
    console.clearScreenBefore();
  }

  console.write('\r');
  console.moveCursorDown(2);
}

After this change we get the expected result:

> dart clear.dart clearLineAfter
#########
####
#########
> dart clear.dart clearLineBefore
#########
     ####
#########
> dart clear.dart clearScreenBefore

     ####
#########
> dart clear.dart clearScreenAfter 
#########
####