[Feature] scan indented blocks (blockquotes and lists)

[mwittie]

Describe your changes

Extends the block scanning logic to handle indented blocks — specifically nested blockquotes and list items. The parser now tracks the indent context (e.g. > , - ) as it traverses the tree-sitter AST, strips those prefixes from block content, and attaches the computed indent string to each Block.

Key changes:

• Block gains an indent field (the nesting prefix, e.g. > > ) and a continuation field (whether the block starts on the same line as the preceding one).
• MakeBlocksFromMarkdown is restructured into a blockCollector that recurses into block_quote and list_item nodes, correctly stripping and forwarding indent prefixes.
• Inline HTML comments are split out of text blocks in a post-processing pass.
• Unclosed fenced code blocks and HTML comments are now detected and returned as errors.
• Block kind constants are renamed to snake_case (text, fenced_code, html_comment).
• Block.Content() now returns string instead of []byte.

Notes for reviewers

Checklist before requesting a review

If any of these checks are missing, please provide an explanation.

• I have updated README files, if applicable.
• This PR is small, otherwise explain why.
  • This PR is large because it restructures the core block scanning logic; splitting it further would leave the implementation in an inconsistent intermediate state.