fix: narrow the BackoffOptions "type" union

[lbennett-stacki]

Seems the ability to define custom-named backoff strategies has been dropped, so we can narrow the type type.

Related:
#2275

[roggervalf]

hi @lbennett-stacki, we can define custom backoff strategies

[roggervalf]

pls take a look https://docs.bullmq.io/guide/retrying-failing-jobs

[lbennett-stacki]

@roggervalf Thanks, I was getting confused between bull and bullmq custom backoff strategy definitions. I think this is still a valid change that may direct others away from some similar confusion.

In bull you can define a custom backoff strategy in one place and reference it by its unique key in the backoff settings object. However in bullmq you cannot define named/keyed backoff strategies as you did in bull, the value of type in the backoff settings object is now only defined by how it is neither fixed nor exponential, the string value is never used to lookup a strategy. (right???) An explicit "custom" value indicates this better than any string that isn't fixed or exponential IMO.

Let me know if I've misread something!

[roggervalf]

hey @lbennett-stacki, but custom won't be always a valid value, you are restricting to use fixed, exponential and custom specific values, instead of any other names as custom and ts will detect it as an error if someone define a different custom name apart of 'custom'

[roggervalf]

as my comment above, this change will cause an error in ts