esp-idf "pure" CMake

[malachib]

Shift our lib to primarily be a "pure" CMake component under esp-idf. Need not be INTERFACE only like rpi-pico, but if that smooths transition, that's OK too.

This might already be functional, depending on whether esp-idf dependencies within our code can resolve.

Impetus being inclusion of dragonbox float support

NOTE: None of this makes platformio targeting happier, since it doesn't use cmake and dragonbox isn't in their library system. Perhaps we need to embed dragonbox.h

[malachib]

See https://bitbucket.org/malachib/playground.esp/issues/40/pure-cmake-component