cfg(package): mark peer-dependency on `@gkalpak/ng-maintain` as optional
cfg(package): mark peer-dependency on `@gkalpak/ng-maintain` as optional

This commit marks the peer-dependency on `@gkalpak/ng-maintain` as
optional. This prevents older NPM versions from emitting warnings when
the package is not present and newer versions of NPM (v7+) from trying
to install the package.

NOTE: Trying to install the peer-dependency with NPM v7.7.6 fails
      ([example failure][1]).

[1]: https://travis-ci.org/github/gkalpak/aliases/builds/766787677

gkalpak/aliases