Update build for .NET 6.

[tillig]

• Standard build scripts with core Autofac.
• Standard analyzers with core Autofac.
• Implicit usings, file-scoped namespaces.
• Removed code that no longer builds (if #NET461).
• Publish symbols as .snupkg.

Does not include the README in the package because the README probably needs some prettying-up and actual info first.

[codecov[bot]]

Codecov Report

Merging #45 (775ae71) into develop (04bebdf) will increase coverage by 6.53%. The diff coverage is 77.57%.

@@             Coverage Diff             @@
##           develop      #45      +/-   ##
===========================================
+ Coverage    74.26%   80.80%   +6.53%     
===========================================
  Files            3        1       -2     
  Lines          136      125      -11     
  Branches        25       24       -1     
===========================================
  Hits           101      101              
+ Misses          23       14       -9     
+ Partials        12       10       -2     

Impacted Files	Coverage Δ
...ofac.Extras.DynamicProxy/RegistrationExtensions.cs	80.80% <77.57%> (+4.18%)	:arrow_up:

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[tillig]

Good idea. I'll get another PR in here for that.