Recently in #15468 the name of the scripts task used by the LavaMoat
policy generation script was renamed from `scripts:prod` to
`scripts:dist`, but we neglected to change this name in the LavaMoat
policy generation script itself.
The script task has now been updated so that the script works again,
and the LavaMoat policy generation script has been re-run.
This PR converts `generate-lavamoat-policies.sh` to `.js` using Yargs. This makes it easier to only generate policy files for a specific build type (using the `-t` flag), which is often useful during Flask development. In addition, the `lavamoat:background:auto` scripts are renamed, and the main readme is updated with some useful tips.
Note that `lavamoat:background:auto:dev` is removed and `lavamoat:background:auto` should be used during local development.
