break out tweakable parameters into a separate file

this allows for:

include <parameters.scad>
// override a variable such as
frame_z = 60;
include <components.scad>

which will let individual components tweak settings, e.g. the height of
a frame

src/top-panel-inset-lever.scad

@@ -16,6 +16,7 @@
  * the Buildable Stick System. If not, see <https://www.gnu.org/licenses/>.
  */
 
+include <parameters.scad>
 include <components.scad>
 
 // combine the lever mount and generic plate, with mounting holes