add ndjson-uniq

[ItsNickBarry]

I've added a new command: ndjson-uniq. It returns the first value with any given result when passed to an expression. It's a lot like ndjson-filter, but I don't know of a way to get the same behavior out of the existing commands.

[ItsNickBarry]

I found another way to do this:

ndjson-reduce 'p.set.has(d) || p.results.push(d), p.set.add(d), p' \
'{ set: new Set(), results: [] }' | \
ndjson-split 'd.results'

It's a bit inconvenient. I'm going to keep using ndjson-uniq.