[JS] Read and write int64/uint64 as strings in integration test JSON format

[asfimport]

Reporter: Wes McKinney / @wesm

Related issues:

• [Developer] Tracking issue: use strings for 64-bit int64 and uint64 in integration test JSON format (is a child of)

_{Note: This issue was originally created as ARROW-6516. Please see the migration documentation for further details.}