v0.2.1

The store pages. Nothing in the stack's behaviour changes.

• VS Code Marketplace: a new page for the extension, with screenshots, a
  short feature list, how to get started and the requirements; a changelog;
  categories and keywords that match what the extension does.
• npm: a plain description for every @protoboardai package, a README
  for the command line (@protoboardai/cli) and for the packages that had
  none.

Every package and the extension move to 0.2.1 together. A 0.2.0 project
needs no change: it asks for ^0.2.0, which 0.2.1 satisfies.