include caching of libraries
test github actions syntax magic as well

replace with ${GITHUB_OUTPUT}
https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/

placeholders do not really work for more than one line

https://docs.github.com/en/communities/using-templates-to-encourage-useful-issues-and-pull-requests

store objcopy of these extra symbols and .map from the linker
distribution will have an extra debug .zip file that could be used with
the exception decoder

also fixes relative paths in the build-and-copy
(although, yet again loses the pio target in the gui)

Resolve the issue with the UnixHostDuino not really being compatible
with the esp8266 Core String (...and the rest of the Core, as well)

Port the CMakeLists.txt from the rpnlib and update it use FetchContent
instead of either manually fetching dependencies or using PIO artifacts
Caching is *expected* to work, but might need slight adjustments

As noticed in logs for 'Post Cache PlatformIO'
> Unable to reserve cache with key ..., another job may be creating this cache.

And update other options to support y, yes and true

- remove .travis.yml
- ./travis_{script,install}.sh -> ./ci_{script,install}.sh
- flush stdout in test script to see logs more promptly
- adjust git config to ignore 'detachedHead' warnings