fix: handle symbol in validateLanguageOptions

[lumirlumir]

Prerequisites checklist

• I have read the contributing guidelines.

AI acknowledgment

• I did not use AI to generate this PR.
• (If the above is not checked) I have reviewed the AI-generated content before submitting.

What is the purpose of this pull request?

Which language are you using?

CommonMark and GFM.

What did you do?

When I use MarkdownLanguage#validateLanguageOptions with a symbol value, it produces an incorrect error message.

What did you expect to happen?

I expected the error message to be:

Invalid language option value `Symbol(frontmatter)` for frontmatter. Expected one of `false`, `"yaml"`, `"toml"`, or `"json"`.

What actually happened?

However, I got:

'Cannot convert a Symbol value to a string'

Link to Minimal Reproducible Example

Create a test.js file in the root of the markdown project.

import { MarkdownLanguage } from "./src/language/markdown-language.js";

const language = new MarkdownLanguage();

for (const languageOptions of [
	{ frontmatter: Symbol("frontmatter") },
	{ math: Symbol("math") },
]) {
	try {
		language.validateLanguageOptions(languageOptions);
	} catch (error) {
		console.error(error.message);
	}
}

What changes did you make? (Give an overview)

This is a simple bug fix. I’ve coerced the message variable with String so that it can also handle an incorrectly passed symbol value.

This is a subtle edge case, but I noticed it while working on #706.

Related Issues

N/A

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

I’ve verified that the JSON and CSS language plugins don’t have this type of issue.

[DMartens]

The JS, CSS and JSON language do not have the problem as they do not print the value.
I think the explicit String cast only makes a difference for Symbol and as such I think the change is fine.
Leaving open for a potential second review.