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.
