Improve MD060 wide-character coverage (arrows, rare CJK, fullwidth forms)

[seeones]

Background

MD060 (table-column-style) computes each table column's visual width with the string-width package. In practice, arrows such as ↑ ↓ ← → (U+2190–U+21FF) and some wide characters outside the common CJK ranges (rare ideographs like 𠀀, 〇, vertical forms, fullwidth signs, etc.) are not always measured as double-width, so "aligned" tables containing them are still reported/mis-fixed.

Changes

• lib/md060.mjs: replace the string-width import with a local visualWidth() helper — grapheme-cluster count via Intl.Segmenter (Node ≥16, well within the required Node ≥22) plus an explicit UAX Rule MD004 incorrectly flagging on an Ordered List #11 Wide/Fullwidth character class. The class additionally covers arrows (U+2190–U+21FF), which are "Ambiguous" in UAX Rule MD004 incorrectly flagging on an Ordered List #11 but render double-width in CJK contexts/terminals.
• test/table-column-style-wide-characters.md: add Arrows and Rare CJK sections; test/markdownlint-test-scenarios.mjs.snapshot updated accordingly.

To keep this PR minimal, package.json is intentionally untouched — string-width is no longer imported anywhere and its dependency entry can be dropped in a follow-up at the maintainer's discretion.

Testing

• node --test test/markdownlint-test-scenarios.mjs (408 tests) and the remaining snapshot suites (119 tests) pass.
• eslint --max-warnings 0 lib/md060.mjs passes.

Related

The same width-calculation approach is used by the table formatter in vscode-markdown (Markdown All in One), which had the identical coverage gap. A corresponding fix has already been submitted there and is open for review:

• PR: Fix table formatter width for arrows and rare wide characters yzhang-gh/vscode-markdown#1581

[DavidAnson]

If the string-width package is handling these characters incorrectly, please open an issue with that project. That will improve all projects that use it vs. requiring separate changes everywhere.