Make default worktrees pattern for content sources configurable

As a follow-up to #743 (closed), and to be consistent with the branches and tags patterns, make the default worktrees pattern configurable. The default value gets used when the content source is missing the worktrees key. Currently, the default value is fixed as ['.']. It should be possible to set a different value using the worktrees key on the content key.

content:
  worktrees: true
  sources:
    ...

This example would enable discovery of the main and linked worktrees on all local repositories.