[Grbl_Esp32 PR#644] Use less memory

a) closeFile() now does SD.end() to release memory
after running a file from SD.
b) Several task stacks are smaller
c) All tasks now check their free space if DEBUGREPORTSTACK_FREE
is defined. platformio.ini has a commented-out line that can be
uncommented to turn that on.
d) Similarly, platformio.ini can turn on DEBUGREPORTHEAP_SIZE
e) Fixed a small leak that occurred when listing local files.

With these changes, the heap size tends to hover around 53K, dropping
to about 37K when running a file from SD.