Callback for scratch terminals

[cempassi]

After a process in a scratch terminal is executed, it would be useful to be able to execute a lua function taking the exit_status of the job as it's input.

I think the implementation should be pretty straight-forward as the building blocks already seems to be in place ([jobwait](https://neovim.io/doc/user/eval.html#jobwait())).

I'm thinking about two main api:

• A table field in the scratch paramater: lua require('FTerm').scratch({ cmd = 'yarn build' , on_exit = function})

• A function call( is this possible in lua?): lua require('FTerm').scratch({ cmd = 'yarn build' }).on_exit(function)

My main goal is to be able to interact with nvim-notify nicely.

[numToStr]

I haven't looked at it for myself to how to implement this. This plugin only contains a single Lua class that is the base of all the terminal features ie. default, custom, and scratch. If this gets implemented then every terminal will get the benefit of it 😄 .

Implementation wise I'll probably go with this

lua require('FTerm').scratch({ cmd = 'yarn build' , on_exit = function(code) end})

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Also, PR is welcome if you wanna give this a shot.