Drop Node 8 support

[feross]

Node 8 is EOL on December 31 (in 5 months from now). This date is earlier than the usual date in April. I believe a special exception was made to EOL Node 8 a bit earlier than normal, just for this version.

If we drop support for Node 8 then we can remove some unneeded dependencies and some hacks in the codebase. I'd like to do that. Folks who need Node 8 can pin their webtorrent version in package.json.

This is part of my overall effort to update and clean up webtorrent dependencies: brave/brave-browser#5490

Some things to do:

• Remove mkdirp and replace with fs.mkdir(path[, options], callback) with the { recursive: true } option.
• Remove require('url') (even for fallback) and replace with URL everywhere

[feross]

0.106.0 is released, without Node 8 support.