feat(cmds/files/ls): recursive listing

[schomatis]

Rough prototype for review. Sharness tests missing.

Follows unix ls -R behavior. Example:

$ ipfs init
$ ipfs files mkdir -p /1/2/3/4

$ ipfs files ls -R 
/:
1

/1:
2

/1/2:
3

/1/2/3:
4

/1/2/3/4:

Closes #8104.

[schomatis]

Ideally we should skip this for the last entry we list. What would be the cleanest way to signal that? Closing the response?