"field" definition allows for undocumented expression

[lachlanmcdonald]

The field definition includes the string definition, which allows for the following expression to validate:

${ a."string" }

I'm not sure whether this is intentional; I can't seem to find any references to this expression in the documentation. Perhaps it should be:

propertyAccess = atom {'.' id}
               | atom {'[' , exprNode , ']'};