feat: add newline option for output line endings

[mir-ashiq]

Adds a newline option to ToStringOptions, allowing stringify() and
Document#toString() to emit \r\n line endings, as discussed in #706.

Following the approach suggested there, the substitution is applied as a
final .replace(/\r?\n/g, '\r\n') over the completed document, which is
safe since line breaks in scalars are normalized to \n when parsing.

• src/options.ts — declare the newline option
• src/stringify/stringify.ts — default value ('\n')
• src/stringify/stringifyDocument.ts — apply the replacement at the final join
• tests/doc/stringify.ts — 5 tests: default output, whole-document
  conversion, Document#toString, directives, and toStringDefaults
• docs/03_options.md — document the new option

Closes #706

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LLM Use

Adding this disclosure after the fact: the code, tests and the original description of this PR were prepared with AI assistance (Claude), and the comment on #706 was AI-drafted as well. The contributing guide's LLM section asks for this to be declared and pre-approved in an issue first; that step was missed here because I looked for CONTRIBUTING at the repo root instead of docs/. Sorry for that.

How to proceed is entirely up to you: if you'd rather not take an assisted PR that skipped the approval step, close it and that's completely fair. If it can continue as declared LLM-assisted work, I can explain every part of the implementation and will keep further issue/PR comments in my own words.

[eemeli]

Closing due to ignoring PR template and unapproved LLM use.