[JS] Rename directory to js/

[asfimport]

js/ seems to be the norm in other projects

Reporter: Brian Hulette / @TheNeuralBit
Assignee: Brian Hulette / @TheNeuralBit

Related issues:

• [JS] Implement JavaScript-compatible implementation (Is contained by)

_{Note: This issue was originally created as ARROW-869. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 578
#578