Add task provider APIs

首先閱讀連結的 VS Code 1.14 task provider 文件，以及 issue 中提到的現有 New-VSCodeTask.ps1 作法。提案也參考了 Register-EditorTask 和 workspace profiles，但沒有找出任何實作檔案或測試。要視為完成，需要一個已達成共識的 API，以及一份用於從建置指令碼和其他 provider 註冊 task 的具體實作計畫。

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?