[feat] compose should handle deeply nested arrays

## Describe the feature

https://github.com/koajs/compose/blob/9a2a426b32c614835b812ecb8de5af06c6c87f6f/index.js#L31

`.flat()` takes `depth` parameter, which defaults to 1 https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/flat. Developers should expect it to be `Infinity` as currently this function would (probably, I haven't checked) break with an argument like `compose([ [fn1, fn2] ])`

this was not supported before the introduction of `.flat()` so is not a bug

Start at index.js line 31 and inspect how compose flattens middleware arrays. Verify that deeply nested input such as compose([[fn1, fn2]]) is accepted, and confirm completion by covering the behavior in the repository's existing tests.

Written by the indexing model from the issue text.