Provide func host start as a task

[ejizba]

...rather than hard-coding it in every user's project. It provides the following benefits:

1. We can update the task under the covers as necessary without having to change a user's project
2. We write less stuff to a user's launch.json and tasks.json

There's two issues in VS Code that affect this experience:
microsoft/vscode#54397
microsoft/vscode#63874

[ejizba]

Also need this bug fixed:
microsoft/vscode#66561