Feature: Support importing ndjson files using abc import

Currently, only `jsonl` formatted JSON files are supported by `abc import`, however many applications use `ndjson` formatted output. The `ndjson` format can be found [here](http://ndjson.org/).

##### Checks

- [ x ] This issue is not a duplicate.
- [ x ] This issue has a description.

Start with the abc import command and compare its current jsonl handling with the ndjson specification linked in the issue. Trace where input format detection and record parsing occur, then verify the command accepts ndjson files and imports their records successfully. Done means ndjson input works without regressing existing jsonl imports.

Written by the indexing model from the issue text.