Add leading-args option for function-arrows

[brandonchinn178]

Blocked by #247
Resolves #233

[github-actions]

👋 @brandonchinn178
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Reviewer: Please verify the following things have been done, if applicable.

• A file has been added to changelog.d/
• "Configuration > Available options" section in README.md has been updated
• "Configuration > Specifying configuration" section in README.md has been updated
• fourmolu.yaml updated to stay in sync with config in README.md
• Tests have been added

[brandonchinn178]

The original issue suggested that this first argument would be indented like

   String
-> String
-> a

but it wasn't trivial to implement that exactly, and I don't see it as a big deal. If a first arg becomes the second arg, you have a diff either way:

-String
+Int
+-> String
 -> String
 -> a

-   String
+   Int
+-> String
 -> String
 -> a

[brandonchinn178]

cc @tchoutri would love to get your feedback here, given that you commented on the original issue.
cc @georgefst would also be interested in your thoughts

[georgefst]

I don't ever intend to use this, so I won't comment on output. But the implementation looks fine.

[brandonchinn178]

Merging now, we can improve upon this later. (Specifically, would like to improve #269)