[JS] wrong signed to unsigned conversion in js

[asfimport]

In JavaScript reader, signed integer vectors are initialized with unsigned int vectors. This is in file types.ts, lines 158, 159, 160 and 167, 168, 169.

Reporter: Saman Amraii
Assignee: Paul Taylor / @trxcllnt

_{Note: This issue was originally created as ARROW-1370. Please see the migration documentation for further details.}

[asfimport]

Brian Hulette / @TheNeuralBit:
This issue is referencing the old implementation, and has been resolved in the new one