nextResult/nextError return the internal state, not the spy, so chaining throws #60

Spy declares both as returning this:

nextError(error: any): this
nextResult(result: R): this

but populateSpy returns state from each:

https://github.com/tinylibs/tinyspy/blob/main/src/internal.ts

defineValue(spy, 'nextError', (error: any) => {
  state.next.push(['error', error])
  return state
})
defineValue(spy, 'nextResult', (result: R) => {
  state.next.push(['ok', result])
  return state
})

The internal state has no nextResult/nextError of its own, so the chain the types promise compiles and then throws:

const s = spy(() => 'real')

s.nextResult('a').nextResult('b')
// TypeError: s.nextResult(...).nextResult is not a function

console.log(s.nextResult('a') === s) // false

Calling them as separate statements works fine, so this only bites chaining.

willCall in internalSpyOn does return spy, which is presumably the intent here too — happy to send a PR making these return the spy if that's the direction you'd want.

Found while writing the API reference in #59; I left chaining out of the docs because of this.