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" nil 10)

;; Or simply,

(source "gnu" 10)

[jcs090218]

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