[7.0.4] parse error caused by renaming `stack` variable

[pnck]

POC:

%%{
    machine M;
    variable stack stack;
    B := 'B' @{fret;}
    ;
    main := 'A' @{fcall B;}
    ;
}%%

fn f() {
    %% write exec;
}

---

• stack is the only variable triggering this issue.
• all language host using .ri files (e.g. go,java) are affected.
• using a fresh build from master branch (both colm and ragel).