jv_parse: let decNumberFromString/strtod parse complex nans as a NaN

[emanuele6]

Before this patch (when using decNumber), "Nan123" was parsed as a NaN,
only if the first n was uppercase.

[emanuele6]

Initially, I also added this test:

# also "nan with payload" #2985
fromjson | isnan
"nan1234"
true

But then I removed it because I realised "nan with payload" is parsed differently on the non-decnumber build (nan(0x4D2)), so the test would be decnumber specific.

Now I realise that many other tests in jq.test will only work in decnumber builds; we should probably move them to another file like we do for onig.test.

[wader]

Now I realise that many other tests in jq.test will only work in decnumber builds; we should probably move them to another file like we do for onig.test.

Yes probably a good idea to split them out. We do have https://github.com/jqlang/jq/blob/master/.github/workflows/decnum.yml but it seems to not run the standard tests... maybe it should?

[wader]

Should we add some kind of tests also?

[emanuele6]

It used to have tests, but I removed it because it would be decNumber specific; I guess I could revert it for now since other tests in shtest are decNumber specific

[wader]

Yeap i think that would be good. After release we can maybe split out things and also make the decnum action (that builds without decnum support) run tests