Sub name matches filename with extension .macro.
Added core event for handling special gcode comments used by expressions and flow control: DEBUG, PRINT and ABORT.
Added overrideable default $-setting values for second PWM spindle to grbl/config.c.
Added clear of return value on CALL statement and added optional return value expression support to ENDSUB.
Improved handling of multiple simultaneous spindles. Still work in progress.
For developers: moved user M-code entry point hooks from the HAL structure to the core handlers and changed signature of the check() function to better support valueless words (letters) when parameter support is enabled. Removed deprecated parameter from the validate() call signature.
Added $384 setting for controlling G92 offset persistence.
Improved $help command output and handling.
Moved the optional tool table in non-volatile storage.
Added gcode parameter support and optional expression support.
