Pass-through for formatter/page_width in analysis_options.yaml

[isoos]

Fixes #1405

[munificent]

FYI this could also be:

  if (origMap['formatter'] case {'page_width': int pageWidth}) {
    final customFormatter =
        customMap.putIfAbsent('formatter', () => <String, Object?>{}) as Map;
    customFormatter['page_width'] = pageWidth;
  }

[lrhn]

Or even

if (origMap['formatter'] case {'page_width': int pageWidth}) {
        (customMap.putIfAbsent('formatter', Map<String, Object?>.empty) as Map)['page_width'] = pageWidth;
  }

(But I guess the extra variable was for readability.)

[jonasfj]

@lrhn only if you use dart format with a page width of at-least 103 chars 🤣

Otherwise it becomes (once formatted):

  if (origMap['formatter'] case {'page_width': int pageWidth}) {
    (customMap.putIfAbsent('formatter', Map<String, Object?>.empty)
        as Map)['page_width'] = pageWidth;
  }

Personally, I'd suggest it's more readable (albeit a bit dumb) to do:

  if (origMap['formatter'] case {'page_width': int pageWidth}) {
    customMap['formatter'] ??= <String, Object?>{};
    customMap['formatter']['page_width'] = pageWidth;
  }

or

  if (origMap['formatter'] case {'page_width': int pageWidth}) {
    customMap['formatter'] = {
      ...?customMap['formatter'],
      'page_width': pageWidth,
    };
  }

Arguably, they aren't as efficient, but they read nicely -- which is sadly almost always the most important thing (at-least in pana).

(for the record, I have started using records for matching JSON, it's awesome)