Draft NDJSON support

[mstackhouse]

I had claude take a crack at support for NDJSON. This isn't going to address performance, specifically for writing, because it's bypassing yyjsonr's implementation. But I'm thinking that I can try contributing to yyjsonr to allow it to write out ndjson rows for dataframes the way that we need it. Once that's done, we could alter the implementation to use the yyjsonr defaults, or just wait to merge this then.

Here's an example of using the current implementation:

library(datasetjson)
ae_orig <- read_dataset_ndjson("https://github.com/cdisc-org/DataExchange-DatasetJson/raw/refs/heads/master/examples/sdtm/ae.ndjson")

infile <- tempfile()
write_dataset_ndjson(ae_orig, infile)

ae_comp <- read_dataset_ndjson(infile, chunk_size=10)

diffdf::diffdf(ae_orig, ae_comp)
#> No issues were found!

^{Created on 2026-02-04 with reprex v2.1.1}