Package as .NET tool

[hickford]

Package as .NET tool https://docs.microsoft.com/en-us/dotnet/core/tools/global-tools https://docs.microsoft.com/en-us/dotnet/core/tools/global-tools-how-to-create

First step for #820

Tested locally

dotnet pack --output out/nupkg
dotnet tool install --global --add-source ./out/nupkg  git-credential-manager-core --prerelease

Package size is 1.3 MB, installation size 4.7 MB. The UI helpers are not included.

This works nicely.

 which git-credential-manager-core
$HOME/.dotnet/tools/git-credential-manager-core
> git-credential-manager-core diagnose
Running diagnostics...

 [ OK ] Environment
 [ OK ] File system
 [ OK ] Networking
 [ OK ] Git
 [ OK ] Credential storage
 [ OK ] Microsoft authentication (AAD/MSA)
 [ OK ] GitHub API

Diagnostic summary: 7 passed, 0 skipped, 0 failed.