ABP CLI: new command: install-libs

[hikalkan]

This is for MVC / Razor Pages and Blazor Server UI types. For other UI types, just write a warning and exit. It should work in the csproj file's folder.

abp install-libs command internally;

1. Uses npm install to install packages (warns if npm was not installed)*
2. Copies css/js/... resources to the libs folder. This is a replacement of copy-resources.js. That means we no longer depend on gulp for that.

*discussion: should we prefer yarn if available, then fallback to npm install? what do you think @armanozak?

Note: Please also update the docs.

[armanozak]

If the user has yarn v1.20+ installed, yes we (still) should.