Add support for setting custom project file in cabal script-metadata

Hi, in `haskell-ide-engine` there exist [an script to build the project using shake](https://github.com/haskell/haskell-ide-engine/blob/master/install.hs). Actually the script has to be ran with cabal this way:
```
cabal v2-run install.hs --project-file install/shake.project
```
I think @power-fungus mentioned that maybe the cabal script feature could be improved adding the option to set a custom project file in the script metadata and be able to run it simply with:
```
cabal v2-run install.hs
```
adding in the script somewhat like:
```
{- cabal:
build-depends:
base
, hie-install
project-file: install/shake.project
-}
```

I've not found any issue about, sorry if it already exists.

This one is related with #5698 cause you could set all available `cabal.project`-fields in the custom project file.