parser: support for struct field tag

[sbinet]

the following fails in ng:

$> cat tag-struct.ng
type T struct {
    Name string `json:"name"`
}

$> ng ./tag-struct.ng
neugram: parser: expected ",", found "string" (off 42)
neugram: parser: expected "ident", found "string" (off 42)
ng: typecheck: type _ not declared