feat: reduce boilerplate for top-level inherited data

[kszczek]

First of all, I want to thank you for developing this package. It has significantly simplified PDF generation in my Flutter apps and I much prefer using this package over rendering HTML templates to PDF, which I previously did.

However, I’ve encountered a slight usability issue with inherited data. Recently, while working on an invoice PDF, I defined several inherited classes:

class PdfLocale extends Inherited { ... }
class PdfColorScheme extends Inherited { ... }
class PdfAssets extends Inherited { ... }
class PdfInvoiceData extends Inherited { ... }

I wanted to make all of this data accessible throughout the document’s widget tree, but I found that it required a lot of boilerplate. Currently, each data object must be wrapped in an InheritedWidget leading to deeply nested structures like this:

Page(
  build: (final pw.Context context) => pw.InheritedWidget(
    inherited: colorScheme,
    build: (final pw.Context context) => pw.InheritedWidget(
      inherited: assets,
      build: (final pw.Context context) => pw.InheritedWidget(
        inherited: data,
        build: (final pw.Context context) => pw.InheritedWidget(
          inherited: locale,
          build: (final pw.Context context) => PdfInvoice(),
        ),
      ),
    ),
  ),
)

And it only gets worse when using MultiPage, as every individual widget needs to be wrapped in the same manner.

To address this, I’ve introduced a new inherited data list for both Page and MultiPage, allowing inherited data to be inserted directly into the page’s base context. This significantly reduces boilerplate:

Page(
  inherited: [colorScheme, assets, data, locale],
  build: (final pw.Context context) => PdfInvoice(),
)

I’d love to hear your thoughts on this approach and any further improvements needed to get this merged.