Support for VS Code tasks background beginsPattern/endsPattern

[segevfiner]

VS Code supports monitoring background watch tasks by defining a regex pattern to match a message line when compilation starts and another one for when compilation ends. Messages between those two lines will be checked for problems using the problemMatcher patterns. This allows it to pick up problems for the problems view and mark whether the background watch task succeeded or failed, it has a canned problemMatcher configuration for tsc --watch which can be used to see how this should work. (Also VS Code's own committed tasks for watching VS Code own source also have such patterns defined The problem is that @soda/friendly-errors-webpack-plugin doesn't output any message after it goes idle, and after the reported problems, instead you get the compilation ended message before the ES Lint/Build problem report, and no specific message when the background TS type checking ends. There needs to be a messages to anchor the beginsPattern/endsPattern so as to properly support VS Code problem matchers.

See https://github.com/vuejs/vue-cli/issues/7073