Fix implementation for node v21

[airhorns]

On top of #8. Fixes #6.

This fixes support for the latest nodejs version (node 21). Before this change, the tests failed with this kind of message:

/Users/airhorns/Code/stream-shift/index.js:16
    return state.buffer[0].length
                           ^

TypeError: Cannot read properties of null (reading 'length')
    at getStateLength (/Users/airhorns/Code/stream-shift/index.js:16:28)
    at shift (/Users/airhorns/Code/stream-shift/index.js:6:99)
    at Test.<anonymous> (/Users/airhorns/Code/stream-shift/test.js:24:10)
    at Test.bound [as _cb] (/Users/airhorns/Code/stream-shift/node_modules/tape/lib/test.js:89:17)
    at Test.run (/Users/airhorns/Code/stream-shift/node_modules/tape/lib/test.js:108:7)
    at Test.bound [as run] (/Users/airhorns/Code/stream-shift/node_modules/tape/lib/test.js:89:17)
    at Immediate.next (/Users/airhorns/Code/stream-shift/node_modules/tape/lib/results.js:151:7)
    at process.processImmediate (node:internal/timers:478:21)

Node.js v21.3.0

Node made some major changes to the _readableState object for performance here: https://github.com/nodejs/node/pull/50341/files#diff-040c1f5a53844e600d40b33c4624f1fe39fcf2f8d62c76ca3fc5ea5442231469 which broke this package's inspection. buffer[0] is no longer always present, but if it is, it is an array who's .length we can use.

See tests passing here: gadget-inc#2

[airhorns]

Tried this in prod at my company and it works just fine!

[mafintosh]

if (state.buffer.length > 0 && state.buffer[0])

[mafintosh]

Thanks, added a comment, otherwise LGTM

[jamie-phlo]

Any ETA on when this will be merged/released at all?

[mafintosh]

Landed in master

[mafintosh]

1.0.2