Bare autolink positions

Maps GFM bare autolinks from their end rather than their reported start
column, fixing two ways cmark gets the start wrong:

- after a soft line break the start is an offset into the paragraph's
  content buffer, so only a suffix of each URL was highlighted;
- a URL opening a paragraph is reported at column 0, so it was not
  highlighted at all.

The text nodes around the link are repaired with it.

Link label positions

Classify links by source syntax rather than a label-vs-destination string
guess, which restores the opening [ on inline links whose label is a suffix
of their destination ([harbor](#harbor)), stops angle autolinks being shifted
right, and repairs the text nodes around a bare autolink.

Adds Link.kind, Link.labelPosition and Image.labelPosition so callers can
style a link's label separately from its syntax.

Raises the swift-cmark floor to 0.8.0 (already what SPM resolved).

1.3.1 — cmark memory/leak/concurrency/multibyte-position safety harne…

…ss (tests + CI sanitizers; no source change)

Merge pull request #8 from hebertialmeida/swift-cmark

Migrate MarkdownSyntax to swift-cmark

Merge pull request #7 from hebertialmeida/fix-min-version

Bump the minimum requirements for Swift Concurrency

Merge pull request #6 from hebertialmeida/swift-concurrency

Migrate to Swift 6.1

Updating swift-cmark-gfm and fixed warnings

Update swift tool version