Help could show available build tools for script command

Creating a new script requires to select a `BUILD_TOOL`. It would be nice if the available options would be part of the help. Having a look at the source, the options are `stack` and `cabal`.

https://github.com/kowainik/summoner/blob/b56aada330f21a45054bc47c677d7112d44e411c/summoner-cli/src/Summoner/Settings.hs#L60-L63

```
$ summoner script
Missing: BUILD_TOOL

Usage: summoner script BUILD_TOOL [-g|--ghc=GHC_VERSION] [-n|--name=FILE_NAME]
Create a new Haskell script
```

```
$ summoner script --help
Usage: summoner script BUILD_TOOL [-g|--ghc=GHC_VERSION] [-n|--name=FILE_NAME]
Create a new Haskell script

Available options:
-h,--help Show this help text
-g,--ghc=GHC_VERSION Version of the compiler to be used for
script (default: Ghc883)
-n,--name=FILE_NAME Name of the script file
```

Start with the linked Settings.hs lines 60-63 and inspect the help definition for the script command. Run `summoner script --help` before and after the change; done means the help lists `stack` and `cabal` as available BUILD_TOOL options.

Written by the indexing model from the issue text.