a better way of handling plugin generation (credits. @Ryvix)

[erikyo]

As suggested by @Ryvix there was some issue related the generation of the plugin zip file.

In order to fix the issues:

• I've added all the folders as suggested by @Ryvix here
• The block.json was moved outside the src folder, so in this way will not be exluded with the rest of the sources files
• The block enqueue was consolidated, removing the unused index.php inside the src folder
• adds a ".npmignore" inside the /src as per the documentation of packagist says

In order to test this PR, generate a plugin as usual, build the zip file using the "npm run plugin-zip" command, install it and then check if the a default block is available inside the "block editor"

fixes #230

[erikyo]

@Mte90 is it worth it to remove all the index.php located inside the ./assets/src/ folder? Since that folder should not be published I believe they are now a bit useless (as well as creating some unnecessary clutter)

[Mte90]

It will not published if people don't copy the folder and upload it somewhere. I think that to cover all the cases is better to keep that index.php just for safety.