Add prepack/postpack events to scripts

[analogrelay]

After #4407, we have script execution in dotnet. We should add the prepack and postpack events which run before and after the dotnet pack command. Since dotnet pack executes dotnet compile, the flow looks like this:

1. prepack
2. dotnet-compile
   1. precompile
   2. dotnet-compile-csc
   3. postcompile
3. Prepare outputs for packaging
4. Create nupkg
5. postpack

/cc @davidfowl

[TheRealPiotrP]

I think we should generalize this, per @davidfowl's suggestion.

@Sridhar-MS

[analogrelay]

I mostly agree, just want to keep this here temporarily to remember it until we finalize that plan. I'll close it out once we have a clean plan there

[TheRealPiotrP]

This is a duplicate of dotnet/cli#244