AP_Scripting: plane doublet example modification

[Astik-2002]

This pr modifies the plane_doublet.lua example. Added features are A) All four actuators are excited now (aileron, elevator, rudder, throttle) instead of just elevator and rudder B) Aircraft returns to fbwa mode when doublet finishes after the doublet finishes, even if trigger channel is high. This will ensure safety.

[IamPete1]

Our scripting test has picked up a few issues, you can find it here: https://github.com/ArduPilot/ardupilot/actions/runs/8219364128/job/22477236405?pr=26472

Checking libraries/AP_Scripting/examples/plane-doublets.lua 18 warnings

    libraries/AP_Scripting/examples/plane-doublets.lua:17:7: unused variable start_time
    libraries/AP_Scripting/examples/plane-doublets.lua:18:7: unused variable end_time
    libraries/AP_Scripting/examples/plane-doublets.lua:21:7: unused variable MODE_STABILIZE
    libraries/AP_Scripting/examples/plane-doublets.lua:29:7: variable start_time was previously defined on line 17
    libraries/AP_Scripting/examples/plane-doublets.lua:30:7: unused variable end_time
    libraries/AP_Scripting/examples/plane-doublets.lua:30:7: variable end_time was previously defined on line 18
    libraries/AP_Scripting/examples/plane-doublets.lua:31:7: unused variable now
    libraries/AP_Scripting/examples/plane-doublets.lua:34:7: unused variable pre_doublet_mode
    libraries/AP_Scripting/examples/plane-doublets.lua:49:11: shadowing upvalue now on line 31
    libraries/AP_Scripting/examples/plane-doublets.lua:50:11: unused variable pre_doublet_mode
    libraries/AP_Scripting/examples/plane-doublets.lua:50:11: shadowing upvalue pre_doublet_mode on line 34
    libraries/AP_Scripting/examples/plane-doublets.lua:59:15: variable trim_funcs is never accessed
    libraries/AP_Scripting/examples/plane-doublets.lua:60:15: value assigned to variable doublet_srv_trim is unused
    libraries/AP_Scripting/examples/plane-doublets.lua:64:13: value assigned to variable doublet_srv_trim is overwritten on line 91 before use
    libraries/AP_Scripting/examples/plane-doublets.lua:69:13: value assigned to variable doublet_srv_trim is overwritten on line 91 before use
    libraries/AP_Scripting/examples/plane-doublets.lua:74:13: value assigned to variable doublet_srv_trim is overwritten on line 91 before use
    libraries/AP_Scripting/examples/plane-doublets.lua:79:13: value assigned to variable doublet_srv_trim is overwritten on line 91 before use
    libraries/AP_Scripting/examples/plane-doublets.lua:84:13: value assigned to variable doublet_srv_trim is overwritten on line 91 before use

[peterbarker]

@Astik-2002 some cleanup of your commit history here would be good.

Could someone who has previously reviewed this have another look, please?