Display last updated date

[philhawksworth]

• Adds date to content pages
• Adds conditional operations to Lume for expensive tasks, suitable for skipping during dev work.
  Opt in with BUILD_TYPE=FULL env var when running build task
• BUILD_TYPE=FULL flag added to production build task
• deno task serve:full task added for developing with inclusion of expensive operations
• Production and full builds enhance the date from the file creation date to the git last modified date

[ry]

I'd prefer to render this as YYYY-mm-dd

also I think this should take a Date arg rather than string

function displayDate(date: Date): string {
  return date.toISOString().slice(0, 10);
}