feat: allow per-block-type content padding via DefaultTextBlockStyle

[Tseshongfeeshur]

Problem

DefaultTextBlockStyle currently has no way to control the spacing
between a block's content and its own decoration (e.g. the left
border on a blockquote, the background box on a code block). The
only related padding is EditableTextBlock.contentPadding, which is
a single global value shared by every block type in the editor —
there's no per-block override.

As a result, something as simple as "add a bit of vertical breathing
room between the blockquote text and its left border" isn't possible
without reaching into text_block.dart and hardcoding values.

Change

• Add an optional EdgeInsets? contentPadding field to
  DefaultTextBlockStyle, with the corresponding copyWith support.
• Forward it through DefaultListBlockStyle's constructor and
  copyWith override, so list styles can opt in as well.
• In EditableTextBlock, add _getContentPaddingForBlock() (mirrors
  the existing _getDecorationForBlock()) to resolve a per-block
  contentPadding from DefaultStyles.quote / DefaultStyles.code,
  falling back to the editor's existing global contentPadding when
  the block style doesn't define one.

Why this shape

This reuses the existing contentPadding → RenderEditableTextBlock
rendering path (which already separates decoration painting from
content padding) rather than introducing a new spacing concept, so
the actual rendering logic (_paintDecoration, _EditableBlock,
RenderEditableTextBlock) doesn't need to change at all.

Compatibility

Fully backward compatible: the new field defaults to null, and
EditableTextBlock only uses it when a block style explicitly sets
it — otherwise the previous global contentPadding value is used
exactly as before. No existing call sites need to change.

Example

quote: DefaultTextBlockStyle(
  ...,
  contentPadding: const EdgeInsets.symmetric(vertical: 8),
),

Testing

• Verified blockquote/code block render unchanged when
  contentPadding is left unset.
• Verified setting contentPadding on DefaultStyles.quote adds
  vertical spacing between the text and the left border without
  shifting the border itself.