Improve error message when evaluating a query AST

[JosVerburg]

When we evaluate a query AST we don't get as good error messaging as with normal string queries.

I've reproduced this in the ./demo/main.ts by changing the runNormalXPath to

async function runNormalXPath(script: string, asXQuery: boolean) {
	const queryAst = fontoxpath.parseScript(
		script,
		{
			language: fontoxpath.Language.XQUERY_3_1_LANGUAGE,
			debug: false,
		},
		new Document()
	);

	const raw = [];
	const it = fontoxpath.evaluateXPathToAsyncIterator(queryAst, xmlDoc, null, null, {
		debug: true,
		disableCache: true,
		language: asXQuery
			? fontoxpath.evaluateXPath.XQUERY_3_1_LANGUAGE
			: fontoxpath.evaluateXPath.XPATH_3_1_LANGUAGE,
		logger: {
			trace: (m) => {
				traceOutput.textContent = m;
				console.log(m);
			},
		},
	});
	...

I know that I do set the debug flag to false when building the AST, but the evaluate function throws an invalid array length error when I set it to true instead.

Using this, when I execute the following XQuery if ((1, 2)) then '（＾∀＾●）ﾉｼ' else '╰（‵□′）╯' I get the following error message

FORG0006: A wrong argument type was specified in a function call.

While I was expecting the error message as I would get it when treating the query as a string (pass the script to evaluateXPathToAsyncIterator instead of queryAst)

1: if ((1, 2)) then '（＾∀＾●）ﾉｼ' else '╰（‵□′）╯'
   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
2: 

Error: FORG0006: A wrong argument type was specified in a function call.
  at <ifThenElseExpr>:1:1 - 1:43

[DrRataplan]

I think we should just document that we place the query in a comment at the start of the AST (or at least we do in Fonto; just do it in FontoXPath instead). From there we can make harder assumptions it's there and print it in the error.