Update snowpack user types

[FredKSchott]

Changes

• Follow up to an issue brought up in Add a shape to the TS config files #1466 (comment)
• Corrects some issues in our existing "SnowpackUserConfig", where "DeepPartial" wasn't cutting it: some user-facing config options were completely changed to become the easier-to-work-with SnowpackConfig options.

Testing

• TS only change
• partially tested by codebase with an added TODO that would increase confidence even further

Docs

• N/A

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