Adds a new experimental `pulumi watch` CLI command which can be used for inner loop development on a Pulumi stack. This command is only available currently via `PULUMI_EXPERIMENTAL=true` while in active development.
The `watch` command does the following:
1. Watches the workspace (the tree rooted at the `Pulumi.yaml` file) for changes
2. Triggers an `update` to the stack whenever there is a change
3. Streams output containing summaries of key update events as well as logs from any resources under management into a combined CLI output
Part of https://github.com/pulumi/pulumi/issues/3448.
The PULUMI_EXPERIMENTAL flag also makes`query` and `policy` available.
