Value Definition Syntax parser fails on `@contents?`

[tidoust]

The parser throws a SyntaxError: unexpected input when it bumps into an "at-keyword representing the start of an at-rule" (which is supposed to be a valid component value type) followed by a ?:

import { definitionSyntax } from 'css-tree';
const syntax = '@contents?';
const ast = definitionSyntax.parse(syntax);

Using grouping as in [@contents]? (or of course dropping the ? or the @) works, but the syntax looks valid to me, and it now appears in the syntax definition of the @mixin at-rule:

@mixin <function-token> <function-parameter>#? , @contents? )