[Grbl_Esp32 PR#629] $sd/show and handle settings in SD files

The main purpose is to handle settings in SD files, so you can have a SD file that establishes a list of setting values.

As a related helpful tool, there is a new command $Sd/Show=, which displays the contents of on the serial port . Ideally it would display in the WebUI console window too but I haven’t figured that out yet.

One issue with $Sd/Show is that it displays the entire file. That is okay for short files, e.g. ones containing settings or simple gcode test files, but displaying enormous files can be tedious. There should be some option for limiting the number of lines shown.