Add tasks support to devcontainer.json

Looks like, currently, only launch configurations are supported inside devcontainer.json.

You can define launch configurations in devcontainer.json like:

{
  "customizations": {
    "vscode": {
      "settings": {
        "launch": {
          "configurations": [
          ]
        },
      }
    }
  }
}

It would be great to have the same but for tasks. Something like:

{
  "customizations": {
    "vscode": {
      "settings": {
        "tasks": [
        ],
      }
    }
  }
}

This way we could have a set of predefined launch and tasks configurations available in the devcontainer (and checked in the git repository, as they are inside the devcontainer.json file) and allow the user to "extend" those configurations locally (by adding them to .vscode folder that might be added to .gitignore).

NOTE: I suppose this should also be added to .code-workspace settings implementation (as, under the hood, looks like devcontainers are using the workspace settings mechanism) and documented in default settings.