This repo should be archived

[lextm]

Since NuGet packages from this repo only cover ANTLR 4.6 and below and end users should migrate to Antlr4.Runtime.Standard (and Antlr4BuildTasks), I think it is time to

1. Archive this repo on GitHub.
2. (optional) Mark its NuGet packages as deprecated.

Those steps are critical to ensure new users know what are the right things to use.

The actual migration can be rather easy, like this example.