Add minimum NodeJS version requirement to package.json

[mondeja]

Issue: #3959

Minimum version of NodeJS required to install simple-icons will be 0.12.18 after this pull.

[ericcornelissen]

This is okay with me, if it turns out to be incorrect I guess we will find out soon enough 🙃

Nevertheless, here are two language features we use that might potentially be missing from 0.12.18, though i wasn't able to verify this:

• The use of getters in icon objects

  simple-icons/scripts/templates/icon-object.js

  Lines 5 to 7 in 9cee719

  	get path() {
  	return this.svg.match(/<path\s+d="([^"]*)/)[1];
  	},

• The use of enumerable: false in index.js:

  simple-icons/scripts/templates/index.js

  Line 4 in 9cee719

  enumerable: false,

If you can verify this @mondeja that would be great. Otherwise, you can merge this after fixing the requested change and we will find out later whether or not these are (or something else is) a problem.