1.6.2 - add strXxx_T() functions for template programming; fix PrintStr::flush() for ESP32 on PlatformIO

• 1.6.2 (2023-06-25)
  • Add tstrings.h which adds various strxxx_T() overloaded functions
    which take either a const char* or a const __FlashStringHelper*
    parameter.
    • Allows writing C++ template code which is agnostic to whether the
      string argument is in normal memory or flash memory. The compiler will
      select the specific implementation automatically at compile-time.
  • PrintStr.h
    • Add special exception for ESP32 on PlatformIO which seems to be
      stuck using the ESP32 Core from 1.x, which means that its
      Print::flush() is non-virtual instead of virtual (fixed in v2.0.3).
    • See PR#28.