feat (collections): add runningReduce

[ayame113]

last part of #1173

• Did you support importing from mod.ts as well as your functions file?
• Have you made sure to allocate as little memory and do as little steps in your algorithm as possible?
• Did you add/adapt JSDoc comments, add/adapt an example, made sure that the example uses the ts markdown tag and assertEquals to show the result?
• Did you add/adapt your functions documentation in README.md?
• Did you make sure not to mutate the arguments passe to your function?
• Did you add tests ensuring no mutation, empty input and corner cases?
• Are your types flat, meaning there is no unnecessary alias in them that makes your users "go to type definition" twice to understand what to pass?

[LionC]

Thanks for implementing this! Loos good, I just added naming and API comments for consistency with the module. I really like the implementation.

[LionC]

This is actually pretty smart!

I would personally put the function body on a newline, as a lot of things are happening here, the line is hard enough to read as it is and it hides a statements in there.

Suggested change

	return collection.map((el) => _initialValue = reducer(_initialValue, el));
	return collection.map((el) =>
	_initialValue = reducer(_initialValue, el)
	);

[ayame113]

When I apply this change, it doesn't seem to pass the deno fmt check.

[ayame113]

@LionC
Thank you for your review, I have updated the PR.

[LionC]

LGTM! Pinging @kt3k

[kt3k]

@ayame113 LGTM. Nice work!

@LionC Thank you for reviewing in detail