Copy `caddy` installer to create `xcaddy` installer.

[coolaj86]

The work for this is probably as simple as a copy/paste of webi-installers/caddy.

The Cheat Sheet will need updating, but I can probably do that.

See How to Help: Add an Installer

Notes for Cheat Sheet

• [x] which ENVs to try
• [x] how to find modules
• [x] how to add modules
• [x] how to select a version (or branch)
• [x] how to list current config
• [x] how to switch versions

[coolaj86]

Added via https://github.com/webinstall/webi-installers/pull/704

Also tracking https://github.com/caddyserver/xcaddy/pull/159