deps: Update fs-admin to Node-API based version

[DeeDeeG]

• The latest fs-admin was updated to use Node-API instead of NAN. 🎉
• Node-API is better than NAN. 🥇
• We are still using an older version of fs-admin. 🕚
• We should update to newer fs-admin that's based on Node-API. ⬆️

[DeeDeeG]

Note: I used fs-admin 0.19.0 instead of 0.20.0 in core. That's so we can dedupe it with the text-buffer package's indirect dependency on fs-admin 0.19.x.

They are very similar versions of fs-admin. Only two source code lines changed, and two dependency versions were bumped. See: atom/fs-admin@v0.19.0...v0.20.0

We can either accept my PR as-is (deduped fs-admin 0.19.0), accept two different versions of fs-admin in different parts of Atom, or fork text-buffer to bump its fs-admin dependency to 0.20.0. (Or do that last option some time later and merge my PR for now.) Just thought I would mention it and explain myself.

[aminya]

Thanks! We should also update the text-buffer. I remember making some pull requests that did not merge.

[icecream17]

will wait for consensus

[DeeDeeG]

Yeah, wasn't sure if it is supposed to be all 0.19.0 or all 0.20.0, but I can only do all 0.19.0 at the moment.

So, it's all 0.19.0 now. If that's what was requested, then this is ready to go ✔️.