Proposal for Metro plugin architecture

[afoxman]

Working with the Metro team, jointly develop plans for a Metro plugin architecture. Focus on a few "hero" plugins from webpack which have broad appeal and use them to frame the design.

Should it be limited? Or should it be broad enough to support much of the webpack ecosystem (with appropriate adapters)?

The underlying premise is that having plugins will accelerate new capabilities for Metro. Upstreaming to main requires more time and rigor, and exposes everyone to the changes. Plugins are localized to only devs that use them. The tradeoff is that quality may be lower, and Metro performance and caching may suffer as a result.

Output is a jointly acceptable proposal.