Document changes between 0.17.1 and 1.0.0 #1287

Tracking issue to easily document / gather changes in 1.0.0 in a single place.

• upgrade to net461 and netstandard20 - scriptcs executable is now a net461 application, while all scriptcs libraries are netstandard20. The only exception is ScriptCs.Hosting which is also net461
• support for C# 7.3
• update to latest Roslyn (2.8.2)
• Mono / Windows feature parity. 0.17.0 and earlier versions of scriptcs used a custom parser/lexer on Mono, and the Mono.CSharp compiler. In 1.0.0 everything has been unified on top of Roslyn and therefore the behavior is identical on all platforms
• changes to how CLI arguments are used by scriptcs (see Replaced PowerArgs with McMaster.Extensions.CommandLineUtils #1289)
• file system migration functionality has been removed (see Kill FileSystemMigrator #1279). This means there is no more automated copying of bin => scriptcs_bin and packages => scriptcs_packages
• print a better exception when script pack is not available (see print a better exception when script pack is not found #1249)

Minimum requirements for scriptcs 1.0.0: .NET 4.6.1+ or Mono 5.2+.