feat: Add priority as the 3rd argument to source alias

[jcs090218]

Remove source-priority, replace it with

❌ Before

(source"gnu")
(source-priority "gnu"10)

✔️ After

(source"gnu"nil10)
;; Or simply,
(source"gnu"10)

[jcs090218]

Not sure if this is a better approach... 😕