[Library Manager] Add SEN and STC drivers to Arduino Library Manager

[MBjoern]

I would like to add the following libraries to the Arduino Library Manager. They are drivers for Sensirion Environental Sensors.

• https://github.com/Sensirion/arduino-i2c-stc3x
• https://github.com/Sensirion/arduino-uart-sen44
• https://github.com/Sensirion/arduino-i2c-sen44

Thank you

[cmaglie]

Done.