Allow dotnet-something to be more than executable files

### Is your feature request related to a problem? Please describe.
I want to define a `dotnet verify` command that does nothing more than run `dotnet build /p:TreatWarningsAsErrors=true`.

`git` has supports for aliases and it can generally run whatever is called git-something and it's in the path.

### Describe the solution you'd like
It would be cool if we could create dotnet scripts and invoke them as `dotnet something`. the SDK supports EXE files for tools, if it also supported powershell/bash/batch/cmd scripts, it would be super cool.

It would be even better if these scripts could be specified/described as part of the tools manifest or global json, so far I can add it to the repo to share it with my team.

### Additional context
This is probably two issues in one:
- support for invoking non-EXE commands via dotnet-something convention
- support for shareable scripts