Add devcontainer configuration

[GiorgioAresu]

Hi,
thanks for the project, it seems the only one currently able to run on my panlee device.
I struggled to install all the dependencies and make platformio work on my fedora laptop, so I ended up managing to do it with the devcontainers. Here's my contribution back to the community, for once :)

Notes:

• I kept all the default comments, but can strip them out if a cleaner file is preferred
• It automatically created the dependabot, again, can remove it if not needed, but I preferred to ask rather than just omit it
• Added the panlee devices (and instruction on where to find more) in the template, as I didn't understand I had to add them there. In hindsight this was more obvious, but it's my first time messing with platformio, and maybe I'm not the only one

[fvanroie]

Thank you