Fix standalone CR line break handline per YAML 1.2 spec

[Keylan]

Summary

Fixes #595

Treats standalone \r (carriage return) as a line break per the YAML 1.2 specification, which defines line breaks as \n, \r\n, or \r.

Changes

• Lexer: Added findLineBreak() method and updated parsing functions to handle \r as a line break
• CST: Added \r case to tokenType()
• Block scalar resolution: Updated splitLines() regex to split on \r
• Flow scalar resolution: Updated foldLines() and foldNewline() to handle \r

Notes

This PR was developed with LLM assistance.

[eemeli]

Before considering this, my question from #595 (comment) remains relevant:

While recognising that this is a divergence from the spec, does it affect any real-world use for you? As in, do you have some content that is actually using a lone \r as a line break?

If there's no actual content using \r as a line break, there's no reason to make it possible.