fix: handle CR in MarkdownSourceCode and Front Matter util

[lumirlumir]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request?

Which language are you using?

CommonMark and GFM.

What did you do?

@eslint/markdown is unable to recognize CR line endings in the MarkdownSourceCode class, as well as in rules and processors.

What did you expect to happen?

I expect CR line endings to be recognized as valid line endings throughout the Markdown plugins.

According to the Markdown(CommonMark) specification, LF, CR, and CRLF can all be used for line endings.

https://spec.commonmark.org/0.31.2/#line-ending

Link to minimal reproducible Example

Adding the following test case to the tests/language/markdown-source-code.test.js would help identify the problem:

	describe("lines", () => {
		it("should parse CRLF line endings", () => {
			const text = "lumir\r\nlumir";
			const sourceCodeWithCRLF = new MarkdownSourceCode({
				text,
				ast: fromMarkdown(text),
			});

			assert.deepStrictEqual(sourceCodeWithCRLF.lines, [
				"lumir",
				"lumir",
			]);
		});

		it("should parse CR line endings", () => {
			const text = "lumir\rlumir";
			const sourceCodeWithCR = new MarkdownSourceCode({
				text,
				ast: fromMarkdown(text),
			});

			assert.deepStrictEqual(sourceCodeWithCR.lines, ["lumir", "lumir"]);
		});

		it("should parse LF line endings", () => {
			const text = "lumir\nlumir";
			const sourceCodeWithLF = new MarkdownSourceCode({
				text,
				ast: fromMarkdown(text),
			});

			assert.deepStrictEqual(sourceCodeWithLF.lines, ["lumir", "lumir"]);
		});

		it("should parse CRLF CR LF line endings", () => {
			const text = "lumir\r\nlumir\rlumir\nlumir";
			const sourceCodeWithCRLFCRLF = new MarkdownSourceCode({
				text,
				ast: fromMarkdown(text),
			});

			assert.deepStrictEqual(sourceCodeWithCRLFCRLF.lines, [
				"lumir",
				"lumir",
				"lumir",
				"lumir",
			]);
		});
	});

What changes did you make? (Give an overview)

This PR originated from #493. While working on that, I found the scope had become too large, so I split this fix into a separate PR.

In this PR, I've fixed an issue where MarkdownSourceCode and the Front Matter utility could not handle CR line endings.

Notable changes:

• Update MarkdownSourceCode to recognize CR line endings.
• Update frontmatterHasTitle utility to recognize CR and CRLF line endings.

Related Issues

Ref: #493, eslint/css#275

Is there anything you'd like reviewers to focus on?

I'll update the rules and processors to recognize CR in #493. This PR is separate from that one to keep the scope smaller.

[nzakas]

LGTM. Thanks!