Stricter typing for childProcess.all

[ehmicky]

childProcess.all is currently always typed as Readable | undefined.
We could make the types stricter to make it either Readable or undefined depending on Execa's arguments. Namely, it should be undefined only if either:

• the all option is false (the default value)
• both the stdout and stderr option is inherit, ipc, ignore, a Node.js stream or an integer. Or an array with a single item with one of those values.

[ehmicky]

Started at #684.

[ehmicky]

Finished at #689.