Add support for PlatformIO

[martinmoene]

See e.g. PR 64 at optional-lite.

Publishing to the PlatformIO Registry using the pio package publish command:

Preparation of folder to publish/upload:

• Create export of git project tag in separate folder (to omit local non-repo files), e.g. .../00-export/optional-lite-v3.4.0.gz
• Extract .gz to folder .../00-export/optional-lite

Publishing on PlatformIO:

• In VSCode, open PlatformIO .
• Open terminal via Miscellaneous > PlatformIOCore CLI.
• Open PlatformIO local webpage via command: pio home.
• On webpage, right-top click account icon.
• Click GitHub to login via GitHub authentication.
• In terminal, issue Ctrl+c to exit PlatformIO Home.
• In terminal, make sure, the proper default folder is selected (.../00-export/optional-lite)
• In terminal, issue pio package publish.

Todo:

• any-lite:
• atomic-lite:
• bit-lite:
• boolean-lite:
• byte-lite:
• expected-lite:
• jthread-lite:
• observer-ptr-lite:
• optional-fun-lite:
• optional-lite:
• ring-span-lite:
• scope-lite:
• span-lite:
• status-value-lite:
• string-lite:
• string-view-lite:
• type-lite:
• value-ptr-lite:
• variant-lite: