return named tuple (; integral = …, error = …)

Rather than returning a tuple, it would be nice if `quadgk` and friends returned a named tuple `(; integral = …, error = …)`.

This would make the output easier to read, you could do e.g. `quadgk(...).integral` instead of `quadgk(...)[1]`, and it should be mostly backwards compatible because named tuples can also be used just like tuples.

Start by locating the `quadgk` entry point and the related “friends” that return tuples. Check existing coverage for tuple-style results, then verify that results expose `.integral` and `.error` while remaining usable with positional tuple access.

Written by the indexing model from the issue text.