JsArray::new() takes a usize

[dherman]

From the list of remaining breaking changes, this PR implements the one and only remaining case where the Neon API uses a u32 but should use a usize: JsArray::new().

I did an audit of all other occurrences of u32 in the codebase and all the remaining instances are fine:

• Object indexed properties -- the JS semantics explicitly defines this for u32 only
• JsArray::len() -- Node-API exposes this as u32
• ref/unref -- Node-API exposes this as u32
• Node-API version -- Node-API exposes this as u32
• Finalize implementation for u32 -- this is just predefined on a bunch of Rust primitive types
• typed arrays -- u32 is one of the supported typed array types
• lifecycle instance ID -- implementation detail