Fix 4324 - add neovim 0.8 test (#4325)

* Fix 4324 - add neovim 0.8 test
* Update vim testbed image
* Update documentation

Co-authored-by: w0rp <w0rp@users.noreply.github.com>

.github/workflows/main.yml

@@ -29,7 +29,7 @@ jobs:
           - '--vim-80-only'
           - '--vim-90-only'
           - '--neovim-02-only'
-          - '--neovim-07-only'
+          - '--neovim-08-only'
           - '--linters-only'
     steps:
       - uses: actions/checkout@v2