jimtcl: add meta.mainProgram attribute (#445999)

pkgs/by-name/ji/jimtcl/package.nix

···
     homepage = "http://jim.tcl.tk/";
     license = lib.licenses.bsd2;
     platforms = lib.platforms.all;
+    mainProgram = "jimsh";
     maintainers = with lib.maintainers; [
       dbohdan
       fgaz