feat: block-level attributes and single-child block component fold

[farnabaz]

Summary

Two related improvements to block-level attribute handling.

1. Attach a trailing {attr} to the nearest block element. A {attr="value"} at the end of a line now attaches to the enclosing block — paragraphs, headings (h1–h6), bullet/ordered/task list items, and blockquote paragraphs:

A paragraph {style="color: red"}

## Sample Markdown Title {#id}

- a list item {.highlight}
- [x] a task {data-done}

> A note {.note}

The attributes only lift to the block when they sit at the very end of the line, so A {x} paragraph stays literal and **bold**{.x} keeps targeting the <strong>. List-item attributes attach to the listItem, not its inner paragraph. Auto-unwrap lifts an unwrapped paragraph's attributes onto its container (container wins).

2. Fold ::ul / ::ol / ::table / ::blockquote into their single same-tag child. These elements have no native trailing-attribute syntax, so the matching block component is now a first-class way to attach attributes to them. When the wrapper contains exactly one same-tagged child, it folds into that element and applies its attributes directly (outer attrs win), instead of producing a nested duplicate:

::ul{.menu}
- item 1
- item 2
::

parses to a list carrying {.menu} and round-trips back to the ::ul{…} form. Same for ::ol, ::table, and multi-paragraph ::blockquote; single-content blockquotes round-trip to the natural > text {attr} form.

Implementation spans the attribute tokenizer (accept trailing { after text), from-markdown (block attachment, unwrap lift, AST fold), and to-markdown (suffix + wrapper serialization, composing with gfm's table/listItem serializers rather than clobbering them).

Closes #122

[pkg-pr-new]

npm i https://pkg.pr.new/remark-mdc@150

commit: 4a9d49d