Allow IDEs to recognize generated code

[avantgardnerio]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

Presently, prost generates code into the target/ folder, which causes IDEs such as CLion to not be able to go to definition, or provide any auto-complete for this code, making it hard to work with.

Describe the solution you'd like

I'd like to move the generated code under the src/ folder, but .gitignore it.

Describe alternatives you've considered

• Leave as is
• Check in the generated code

[alamb]

I believe these changes were also included in #3311

[tustvold]

FWIW I use CLion with generated code without issue, the trick is to run the build once, and then "Refresh Cargo Projects". You need to run the build first in order to actually generate the code