Parser does not accept if or each block argument in parentheses

[Conduitry]

Attempting to parse {{#if (condition)}}...{{/if}} results in an Expected }} parsing error at the ). And attempting to parse {{#each (list) as item}}...{{/each}} results in an Expected as parsing error at the ).

[Conduitry]

Looks like this affects regular mustaches as well. {{(a)}} fails to parse.

Not sure whether this is a bug in Acorn, or just something about their API I'm not understanding. acorn.parseExpressionAt('{{(a)}}', 2) returns { type: 'Identifier', start: 3, end: 4, name: 'a' }. I suppose this makes sense from Acorn's point of view, but it's a bit unfortunate in this case.

[Conduitry]

This can be addressed with the { preserveParens: true } option in Acorn. This doesn't appear to have any ill effects. I'll submit a PR shortly.