Add task provider APIs

VS Code 1.14 includes a new task provider concept which allows extensions to dynamcially register tasks that show up in the "Tasks: Run Task" menu. I think this would be really useful for the following scenarios:

• Generating tasks from psake or Invoke-Build scripts (@nightroman)
• Generating tasks for Pester tests if any are detected in the project
• Generating tasks for PSGallery operations (publish, etc)

However, I think that generating tasks from build scripts is probably what people would be doing 99% of the time.

@nightroman, I'd love to work with you to get Invoke-Build hooked up with this for the next PowerShell extension update. Here's what I'm thinking:

• Editor Services will provide new cmdlets for registering "editor tasks" with a syntax like this:

  Register-EditorTask -Name "Build" -Source "Invoke-Build" -Command "Invoke-Build .\build.ps1 Build"

• Invoke-Build will provide a new command (very similar to New-VSCodeTask.ps1) which will check for the existence of Register-EditorTask (or PSES being loaded) and then register all available tasks in the editor using the aforementioned command

• The user will create a workspace profile which will invoke your command on their build script so that the build tasks are auto-registered when they load the project in VS Code.

How does that sound? Can you see any problems with this approach? Any better ideas?

Start by reading the linked VS Code 1.14 task provider documentation and the existing New-VSCodeTask.ps1 approach mentioned in the issue. The proposal also references Register-EditorTask and workspace profiles, but no implementation files or tests are identified. Done would require an agreed API and a concrete implementation plan for registering tasks from build scripts and other providers.