fix: allow newlines inside bracket link path

[LoganBarnett]

Summary

Emacs org-mode treats bracket links as inline objects bounded by their
enclosing element (paragraph, list item, etc.), not by a single line. A
[[path]] or [[path][desc]] may span a newline inside either part and is
still recognised as one link by Emacs.

orgize previously excluded \n from the path take_while, so multi-line
links written by Emacs were silently dropped on parse. Description parsing
already permitted newlines, so this PR is a single-character change in
src/syntax/link.rs plus regression tests.

Why this is safe

Element-level parsers (paragraph_node, list item bodies, etc.) already
bound the input passed to link_node — paragraphs terminate at blank
lines, so the input slice handed to the link parser cannot extend past
the surrounding element. Removing the per-link newline restriction
therefore cannot let a link swallow material beyond its element.

Reproducer

The downstream project org-fmt
hit this when round-tripping documents written in Emacs:

let org = orgize::Org::parse(
    "Here is a [[really really long link\nthat just keeps going]] in prose.\n",
);
// pre-fix:  zero links found
// post-fix: one link found

Tests

Three new tests in src/syntax/link.rs:

• parse_multiline_link — link_node directly, covering both
  newline-in-path and newline-in-description cases (snapshot assertions).
• parse_multiline_link_in_paragraph — end-to-end via Org::parse,
  matching the downstream reproducer.

Full suite passes (148 tests), no regressions.

Notes

The commit message references the related precedent (5bc15d8 fix: newline in not allowed in property name) where the project intentionally tightened
newline handling. This change goes the other direction for links, where
Emacs explicitly allows newlines.