lfjs: support b2Contact.SetEnabled and fix b2WorldManifold.GetPoint

[haskasu]

for javascript version.

1. add support for b2Contact.SetEnabled and b2Contact.IsEnabled

2. b2WorldManifold.GetPoint, each point data position interval should be 8 instead of 2

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[haskasu]

I signed it!

[googlebot]

CLAs look good, thanks!

[stewartmiles]

Looks great thanks for the patch.