Fix incorrect field used for aliases from snowpack user config

[ratorx]

Uses the wrong field from userConfig. According to snowpack.config.js documentation, the aliases field is called just alias and not aliases.

I've just changed it in this PR, but it might be worth loading both for now (for backwards compatibility) and dropping the incorrect field in a new release?

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[natemoo-re]

Thank you for catching this! I'm going to merge into the v1.1.0 branch, which I'm hoping to cut a release of as soon as Snowpack drops v3.1.0!