Refactors towards 2-to-6 Linear Axes

[thinkyhead]

Description

The PR #19112 is pretty complicated, so this PR aims to set up in advance the new defines and macros that will be used to support a variable number of linear axes, starting here with the potential to drop the Z axis. Once this PR passes testing and works as expected then it can be merged ahead of the full multi-axis PR, simplifying that PR and more starkly pointing out all the places in the code where changes must go when adding more axes to Marlin.

New Macros

Following Marlin's meta-programming idiom, this PR also introduces macros for lists, arrays, statements, and undecorated (gang) elements, taking the optional E element first, but placing it after linear axes. If EXTRUDERS is set to 0 the E element is ignored.

For example, under the typical setup the following structure will end up with X, Y, and Z fields as linear axes, followed by an E field as the extruder axis. As more axes are added they will go ahead of E. If EXTRUDERS is set to 0, then there will be no E field emitted.

  const abce_long_t target = {
     LOGICAL_AXIS_LIST(
      int32_t(LROUND(e * settings.axis_steps_per_mm[E_AXIS_N(extruder)])),
      int32_t(LROUND(a * settings.axis_steps_per_mm[A_AXIS])),
      int32_t(LROUND(b * settings.axis_steps_per_mm[B_AXIS])),
      int32_t(LROUND(c * settings.axis_steps_per_mm[C_AXIS])),
      int32_t(LROUND(i * settings.axis_steps_per_mm[I_AXIS])),
      int32_t(LROUND(j * settings.axis_steps_per_mm[J_AXIS])),
      int32_t(LROUND(k * settings.axis_steps_per_mm[K_AXIS]))
    )
  };

The "linear axis" macros take a list of values to emit for the linear axes, and only emit items for those that are enabled by the configuration. The "linear axes" are the three cartesian input axes, but might also pertain to the 3 axes needed for Delta, SCARA, etc. The macro requires one per linear axis. So for this iteration these macros require exactly three arguments.

• LINEAR_AXIS_LIST takes a list of things to emit as a list for the linear axes (XYZ or ABC). It must contain 3 items.
• LINEAR_AXIS_GANG takes a list of things to emit with no delimiters for the linear axes.
• LINEAR_AXIS_ARRAY works like LINEAR_AXIS_LIST but emits a complete array of things.
• LINEAR_AXIS_CODE works like LINEAR_AXIS_LIST but each thing is emitted as a line of code.

The "logical axis" macros take a list of values to emit for the logical axes, and only emit the items for those that are enabled by the configuration. The logical axes include the E axis plus all the linear axes, so basically all the axes that you can command from a single line of G-code. For this iteration that means four items.

• LOGICAL_AXIS_LIST takes a list of things to emit as a list for the logical axes (XYZE). It must contain 4 items and the first item must be the E axis item.
• LOGICAL_AXIS_GANG takes a list of things to emit with no delimiters for the logical axes.
• LOGICAL_AXIS_ARRAY works like LOGICAL_AXIS_LIST but emits a complete array of things.
• LOGICAL_AXIS_CODE works like LOGICAL_AXIS_LIST but each thing is emitted as a line of code.

Zero Extruders

Currently when Marlin is configured for zero extruders it still maintains an "E" axis internally. So one aim here is to remove all those unused fields and make the code more strict about when you can or can't refer to an E axis, an E endstop, etc. If EXTRUDERS is set to 0 then the so-called "XYZE" structure will be the same as the "XYZ" structure.