linter: helps minimize diffs by regulating exports

[bjacobgordon]

• enforces separation of exports from their declarations
• enforces placement at the end of files

This has two effects:

• Less keywords surrounding class decorations:
  • after: default export class ... extends ... implements {}
  • before: class ... extends ... implements {}
• Easier to see defaults relate to non-defaults since they're all at the bottom of each file