Refactor and simplify buf.plugin.yaml configuration

[amckinney]

This revises the buf.plugin.yaml according to the feedback in #1210. Specifically, this covers the following:

• Remove archive runtime config
• Simplify runtime configuration handling
• Use modfile.GoVersionRE.MatchString
• Cleanup bufpluginconfig.Config comment
• Use separate runtime configuration shapes (e.g. separate go and npm runtime keys).

Note that there's one TODO left in the implementation surrounding npm runtime dependency validation. For now, we permit any non-empty string for the version. There might be some good solutions for this to capture exactly what npm does [not] support, but that isn't handled for now - we'll need to follow-up here.