Configuration reference interpolation

[kageurufu]

Constants are defined in a [constants] section, and unused constants will only warn instead of erroring.

My intended usage looks like this:

[constants]
run_current_ab: 1.9

[tmc5160 stepper_x]
run_current: ${constants.run_current_ab}
[tmc5160 stepper_y]
run_current: ${constants.run_current_ab}

${option} references the current section
${section.option} looks up anywhere

interpolation occurs at most 10 times right now to avoid infinite loops

Checklist

• pr title makes sense
• squashed to 1 commit
• added a test case if possible
• if new feature, added to the readme
• ci is happy and green

[kageurufu]

As a note, nested lookups do not work right now

${section.${section.name}_name} is awful, but should it work anyway?

edit: now they work. this hurts my soul but it works

[constants]
run_current_xy = 1.9
run_current = run_current
xy = xy

[tmc5160 stepper_x]
run_current: ${constants.${constants.run_current}_${constants.xy}}

[rogerlz]

I believe it implement what has been asked in https://github.com/DangerKlippers/danger-klipper/issues/342

[kageurufu]

I hadn't even looked for the issue. Enjoy @lraithel15133 🙃