feature: allow changing behaviour of devbox init / custom templates

[DerArkeN]

What problem are you trying to solve?

I use devbox init to initalize a new devbox all the time.
Most of the times they consist of same or similar packages, which I have to add manually.

What solution would you like?

I think you should be able to change the default behaviour of devbox init by changing a default file which get's used for creating a new devbox.
One might also consider to be able to create a whole bunch of default configs which can be referred to by devbox init. eg:
devbox init --> default devbox file
devbox init go --> custom user configured go devbox file
etc...

Alternatives you've considered

You might also want to allow custom templates instead doing this with devbox init, but it seems more convenient to do so since the command is shorter.