Close #4401 - Use subtle defaults for virtual-text

Default virtual-text to the Comment highlight group and prefix
virtual-text messages with comment text for each language by default.

Messages can now be formatted with `%type%` to print the error type.

The Vim 9.0 version has been updated in the Docker image to add test
coverage for virtual-text.

Dockerfile

@@ -1,7 +1,7 @@
 FROM testbed/vim:24
 
 RUN install_vim -tag v8.0.0027 -build \
-                -tag v9.0.0133 -build \
+                -tag v9.0.0297 -build \
                 -tag neovim:v0.2.0 -build \
                 -tag neovim:v0.8.0 -build