Target netcore 3.1 (LTS), clean things up

[cartermp]

Hey there, I came across this and got curious about it. Looks like it was a little more complicated w.r.t .NET-isms that it needs to be. Here's what I did:

• Targets .NET Core 3.1, which is the new LTS
• Remove older, unsupported netcore target
• Simplify by not doing multi-tfm. There was no need to do this in any of the projects
• Remove netstandard2.1 target, since none of the APIs from there were used
• Added the .ionide/ folder to .gitignore so VSCode users don't accidentally check in the symbol cache it uses

[ChrisMarinos]

Awesome. Thanks so much for taking the time to contribute this!

[cartermp]

@ChrisMarinos my pleasure 🙂