Composable turbo.json (#2706)
This feature allows monorepos to place a `turbo.json` file in workspaces
with an `"extends"` key to override task configuration for that workspace.
After this PR, individual workspaces should be able to:
- add keys to task defintions (e.g. changing `dependsOn` or `outputs` configs)
- reset keys in task definitions to defaults (`e.g. `dependsOn: []`)
- add tasks (e.g add a `lint` command only in the workspace that implements it)
