Support logging S.Nothing in deno

[jceb]

The current implementation of sanctuary leads deno to show S.Nothing as {}. Example code to trigger the behavior by storing it in file issue.js and executing it with deno run issue.js:

import sanctuary from "https://cdn.skypack.dev/sanctuary@3.1.0?dts";
console.log(sanctuary.Nothing);

The issue stems from Deno not supporting the hook Node.js provides for logging.

Solution: Define a custom inspect function as described here https://doc.deno.land/builtin/stable#Deno.inspect

Deno can be detected as follows: typeof Deno === 'object' && typeof Deno.customInspect === 'symbol'