[ModBuildConfig] Add ability to include arbitrary mods in build output

[abhiaagarwal]

Marked draft because I haven't extensively tested yet.

This adds the ability to include arbitrary mods inside the release zips / mod deploy by specifying <Content> items in ItemGroups. Often, many large SMAPI-based mods have CP-based components that need to be manually copied over requiring an unzip when preparing release zips and/or testing. This makes it a one-step process.