This addresses comments in #1956 and #1079. In particular, this is useful
when both endstops are stationary on a CoreXY system, and the Y axis needs
to be homed before the X so the flags are aligned.
- Add `Conditionals.h` with calculated configuration values
- Add `SanityCheck.h` with checks for configuration errors
- Remove equivalent code from all configurations
- Move error checks from some sources to `SanityCheck.h` also
- Fix initialization of count_direction in stepper.cpp
- Auto-detect “switchable” items where possible
- Handle switchable multi-value fields
- Add standard sections to config form
- Add section markers to configurations
- Add value options to configurations
- Remove and re-add fields for loaded configs
- Preserve sections across configs
- Preserve non-added fields
- Style added fields wider & newline by default