Fix line returns segments being one character too long on Windows

Changes

Previously, due to how we skipped over CRLF, we'd end up with a mapping that looks like this:

Original:  ;    \n
Mapping : [;\r][\n]

Where the first segment would contain both the last character of the line, and the \r. This is fine, albeit wonky, however this ends up breaking downstream in libraries that are line-ending agnostic because they don't expect than an offset would end up after a line ending, even less so in the middle of a two-characters one. (the main culprit here is https://github.com/microsoft/vscode-languageserver-node/tree/main/textDocument, see https://github.com/microsoft/vscode-languageserver-node/issues/1285)

This PR fixes this by handling it like this instead:

Original:  ;  \n
Mapping : [;][\r\n]

That way, asking for the last segment of a line will instead result on the ;, or before the two line endings, depending on which one you ask for, but nothing after or in the middle of it.

Testing

Added a test

Docs

N/A

withastro / compiler