Allow the use as an external module

[lefou]

The current setup is nice to fine tune the checks, but also pretty invasive, as the users need to import and extend missinglink in all of their modules.

When an external module is provided, the user does not necessarily need to import and extend the missinglink plugin in their build.sc to get some checks.

See also:

• Documentation for external modules: https://mill-build.com/mill/Modules.html#_external_modules
• Tasks API, which might come handy to accept a selection of tasks in a command via CLI: https://mill-build.com/api/latest/mill/main/Tasks.html