Add hook_requirements() check for JSONEditor library

Thanks for the great module!
I didn't create a separate service to do requirement check like the main module is doing.
The submodule would also benefit for READMD.md or INSTALL.md help file, eg. exact composer steps how to download latest supported version.
Seems like https://github.com/josdejong/jsoneditor stopped including dist/ to codebase somewhere on 6.x.x.