Consider a "test-adapter-protocol" that maps onto the new testing APIs

I'm a big fan of common protocols like LSP + DAP that reduce the effort for a new language/editor to have support for other editors/languages and have recently been working on implementations for Dart. While migrating to the new VS Code test runner recently, I was wondering if something similar could be done for these new testing APIs. Rather than implementing it directly inside the VS Code extension in typescript, if it could be pushed off to another process it could have some of the same benefits as LSP/DAP (eg. other editors may adopt it, tools can write more code in their native languages, etc.). For Dart, all of the test runs go through the DAP, so the DA is currently forwarding custom JSON events from the `package:test` runner to the extension, and then the extension is translating them into VS Code API calls. It would be much nicer if I could do that translation inside the DA and then other editors could potentially share it. Even as a completely separate protocol, I could forward the events from the DAP straight onto the test protocol client - the useful part is doing the translation in the DAP into a standard protocol that may gain traction like LSP/DAP have. @isidorn @weinand @jacob314