zclsyntax: don't allow quoted strings as attribute names
zclsyntax: don't allow quoted strings as attribute names

This will be allowed for object literals, but attributes are treated
differently to reinforce the fact that they have a predefined structure.

strivecast/hcl,strivecast/hcl,strivecast/hcl,hashicorp/hcl,hashicorp/hcl,strivecast/hcl,zclconf/go-zcl,zclconf/go-zcl,zclconf/go-zcl,strivecast/hcl,hashicorp/hcl,hashicorp/hcl,hashicorp/hcl