feat(portable_text): expose block index to BlockContainerBuilder

[Ortes]

What

Threads the 0-based block index through buildBlock and the default block builder into BlockContainerBuilder and BlockWidgetBuilder.

Why

Block containers currently receive only (context, child), so they have no way to know a block's position in the list. That makes position-dependent styling impossible — e.g. suppressing the leading space above the first heading while keeping it for headings in the middle of the content.

With the index available, a container can do:

PortableTextConfig.shared.blockContainers['h2'] = (context, child, index) =>
    index == 0 ? child : Padding(padding: const EdgeInsets.only(top: 24), child: child);

Changes

• BlockContainerBuilder: Function(BuildContext, Widget) → Function(BuildContext, Widget, int index)
• BlockWidgetBuilder: adds a trailing int index
• PortableTextConfig.buildBlock: takes and forwards index
• PortableTextBlock: new index field (defaults to 0 for standalone use), forwarded to the container builder
• defaultListBuilder passes the ListView.builder index through
• Defaults (defaultBlockContainerBuilder, blockquote, block) updated to the new arity
• README example and tests updated

Breaking change

BlockContainerBuilder and BlockWidgetBuilder now take a trailing int index argument. Existing custom builders need the extra parameter (ignore it if unused).

All package tests pass; dart analyze is clean.