Declare header files for PlatformIO

[ivankravets]

Hi,

Please accept this PR that adds header files according to https://docs.platformio.org/en/latest/librarymanager/config.html#headers

Developers will see them soon in the new version of PlatformIO Package Registry.

Thanks in advance!

[vshymanskyy]

Blynk.h is not meant to me directly included

[ivankravets]

Do you have a list of public headers that can be optionally included in a project? If the headers field is not specified, we will pick the first 25 headers from the source code. We use them to show a quick start guide and for searching missed includes.