Sensor to repeatably determine probe Z offset using PROBE_CALIBRATE Klipper macro

Demo of a 'digital feeler gauge' used to repeatably determine a probe z offset. https://github.com/djsplice/TinyTouch I don't have enough room on my printer to use a sexbolt to enable an auto-z calibration routine. I had moved from using a piece of paper to using an aluminum feeler gauge to help improve the repeatability of setting my probe z offset but found that I still needed to fine-tune my offset using 'baby steps' in order to get a perfect first layer. I believe the variability in measurement using the paper or feeler gauge method is due to the fact that there's some subjectiveness (human feel) involved in determining just how much the nozzle is allowed to pinch the paper or feeler gauge. This subjective measurement can introduce a variance that can affect first-layer consistency. This sensor replaces the paper or fixed-size feeler gauge when using the PROBE_CALIBRATE Klipper macro, and will consistently indicate when the nozzle is touching the bed, generally achieving a consistent z-offset measurement with a variance of +/- .01mm.