Structured listing of known executables to stdout

[brettcannon]

E.g. --json flag for JSONified --list output. Would make consuming data from the CLI easier (e.g. for Nushell).

[brettcannon]

Another way to do this would be to allow --list to take an argument, e.g. utf-8, plain, json, etc. This could help with #83 where something that's easy to parse with e.g. sed would be useful.

[brettcannon]

One way is to have no header and use only whitespace and | to separate things. At least for Nu, that would let you do:

py --list | lines | split column "|" version executable | str trim

[brettcannon]

Tracking this in #432 .