Add dropthis upgrade command for self-updating the CLI

[dmalis]

Add a dropthis upgrade command that checks npm for a newer version and updates the CLI in place.

Expected behavior:

dropthis upgrade
# Checking for updates...
# Updated 0.1.2 → 0.1.3

If already on latest:

dropthis upgrade
# Already on latest version (0.1.3)

Implementation notes:

• Check latest version from npm registry (https://registry.npmjs.org/dropthis/latest)
• Compare with current dropthis --version
• If newer, run npm install -g dropthis@latest
• Consider also showing an update notification on CLI startup (e.g. update-notifier)