builtins: Add lazy generator versions of add/0: add/1 and add/2

[stedolan]

(Github doesn't support moving PRs from one repo to another, so I've copy-pasted the text of @emanuele6 's PR from the deleted prior jqlang/jq repository below. The relevant branch is emanuele6:niceadd )

@emanuele6 says:

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And reimplement add/0 in terms of those.

Also add documentation.

The add/2 version may appear redundant since it is mostly equivalent to add($initial, generator), but it is not really because initial is a $-argument, so add/2 will output a result for each of the outputs of the first expression; consider the following examples:

$ echo 1 2 | jq --jsonarg -n 'add(inputs; $ARGS.positional[])' -- 3 4
8
9
$ echo 1 2 | jq --jsonarg -n 'add(inputs, $ARGS.positional[])' -- 3 4
10
$

$ jq -n '[] | add(.[], 1)'
1
$ jq -n '[] | add(.[]; 1)'
$ # nothing

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If this gets merged, the implementation of walk/1 can be fixed using the fix proposed in #2584.

I think these functions should definitely be builtins since they are very useful to have, it makes sense to have them since they are just a generalisation of the existing add/0.
Also I had to write manual reduce loops almost every day when solving last year's advent of code puzzles in jq because add/0 was too unnecessarily slow/heavy for the large inputs, which felt a bit lame. :/

{min,max}/{1,2} (and maybe also {min,max}_by/{2,3}) should also be added since those would be useful too, but since the implementation of min/0 and max/0 implementation seems to be written in C, I will leave removing and rewriting the C implementation in jq in terms of max/2 and min/2 to another PR.