Fixes a couple of issues with config files in websnacks projects.
First, config files are no longer required and the dev and build
commands will no longer error out if a websnacks.ts/js file doesn't
exist.
Second, all optional user config params are now actually optional -
before some parameters were assumed to exist and would error out if not
present (e.g. the "watch" parameter).
Finally, e2e tests were added to prevent regressions on these issues
and test helpers were extracted to a separate file.
