Improve CLI version checking

[Xeevis]

Related to #1113

Currently version check is working with latest NuGet package and compares equality to assembly file version, this brings up problems in some scenarios.

Like when using newer version MyGet nightly build where 0.17.0 is said to be newer than 0.18.0:

My proposal is to make the following changes:

• Attempt to determine exact installed version by parsing the dotnet tool list -g output, with fallback to existing GetCliVersion()
• Compare versions using the official NuGet.Versioning SemVer implementation to determine the newer one
• If installed version is a preview, compare it against MyGet nightly builds, those are always latest previews
• Preview versions cannot be updated with dotnet tool update. Update unlike Install currently lacks the required --version support Unable to update global tool to the latest prerelease version dotnet/sdk#2551, workaround is to uninstall && install. Should be reflected in the WRN.

Thoughts? I'm willing to give this one a shot if accepted.

[hikalkan]

Thanks for suggestions. If you implement these, please send as a PR.

[hikalkan]

I added this to the next milestone. However, if you implement it, we can include to v0.18.