add library.json to support compilation as platformio library

[rwedler]

This is a simple PR to add compilation support as a platformio library. The only thing I needed to add was a custom option to avoid the usage of Wshadow when compiling littlefs.

To use this in a project, it can be referenced via a URL in the project's platformio.ini with the following text

lib_deps =
    https://github.com/littlefs-project/littlefs#{githash/tag}

[geky]

Hi @rwedler, thanks for creating a PR, I appreciate the thought to upstream this.

First a quick note, to fix the CI failures you would need to rebase/retarget onto the devel branch. GitHub made a breaking change to their APIs somewhat recently and the fix hasn't made its way to our master branch yet.

But w.r.t. this PR, I have a general rule of not adding files that are only necessary to make downstream systems work.

This may feel unfair since it's only one file, but it opens a can of worms and makes it harder to argue against downstream-helpful changes that are unreasonable.

It also makes it so build system changes downstream can backflow into littlefs development. Breaking changes are already bad enough going in only one direction...