Nix supports md5, sha1, sha256, and sha512 hashes. The first two are
deprecated, but many npm packages provide only a sha1, so we have to
allow that one. This commit continues to leverage `ssri` for reliable
integrity string parsing and algorithm selection, but limits it to only
supported hash algorithms.
It also switches the hash representation in the generated nix file to
base64, since that format is supported by nix (suggested by jtojnar).
This saves a little disk space and computation.
Finally, it adds a .prettierignore file, since prettier called with the
arguments in the format command currently tries to format node_modules.
