problemMatcher is too complicated and should be modernised

One of the most frustrating things I've found with setting up a task is the problemMatcher. Its far too complicated for simple tasks such as setting up a background task to the point most don't bother to use it, which is a shame because it's quite powerful. Here are some things worth considering: ## Background Tasks Sometimes I want to have a `watch` task, such as ESBuild watch. This is a pain to configure. 1. `background` should come out of the problemMatcher scope. This is because I don't care about matching the output, I just want to start background tasks and let VSCode know when something is being rebuilt. Even though this is straight forward in theory I now need to define loads of problemMatcher config, and I usually end up just setting `$tsc` as a base, which is wrong if its not a TypeScript Project. This has been brought up as far back as [2016](https://github.com/microsoft/vscode/issues/6209#issuecomment-218154446) by @dbaeumer 2. `background` shouldn't force you to have a `beginsPattern` it makes no sense for watch tasks which start straight away and every subsequent change just triggers similar output (like “build successful”). (see edge case) Edge Case: ESBuild for example [doesn't show any output](https://github.com/evanw/esbuild/issues/980#issuecomment-943321320) when a watch task starts, this means beginsPattern is not applicable, `"activeOnStart": true` is set but a beginsPattern is still required. After the first successful build "watch build succeeded" is shown. I can't set "watch build succeeded" as the beginsPattern because it will never show until the first save, and I can't set it as endsPattern because VScode will think im just beginning a build when it has in fact ended. My Proposal is this: background can stay within problemMatcher for backwards compatibility but it should be a top level item within tasks. It's not tied to problemMatching, and sometimes you just want to mark the beginning and end of a build in watch mode. `beginsPattern` s…