preview2 alignment

[danbugs]

• deleted the .wit.md file, and separated interfaces into unique files inside of the wit/ folder,
• updated files to use 'package management' wit syntax (i.e., pkg, and deps stuff),
• moved err.tracing.{error} into deps in anticipation of previously discussed standardization of error tracing and removed error variant,
• added io dependency in anticipation of message streaming,
• added broker pseudo-resource type, and
• updated the README w/: introduction, goals, and non-goals sections, together w/ a note (like in wasi-http) mentioning the README is still incomplete.