Use .json extension instead of .exe-plugin

[dsherret]

Process plugins currently use the file extension .exe-plugin. I initially went with this decision to emphasize the fact that an executable will be downloaded and executed. Few reasons I would like to change this to use a .json extension instead:

1. I really don't like this custom extension
2. People are forced to provide a checksum anyway
3. The .json extension helps clue to people that this is some manifest file that they can look at.
4. Using the .json extension instead will open the door to using the .json extension for more than just process plugins. Perhaps in the future there could be pre-compiled Wasm plugins that use a json file as well (ex. precompile with llvm, which would be faster) or support for Wasm plugins that need assets in addition to a .wasm file (though, perhaps a zip file could server better).

[dsherret]

Implemented in 489fb79