Set anonymous functions display name using file name

[Ethan-Arrowood]

Related to fastify/fastify#861

When a user adds a fastify plugin with an anonymous name, fp will use look up the stack trace and set the function objects display name to be the file name from which it was called. It stores this in a symbol: fp.DISPLAY_NAME_SYMBOL

For example:

// mySuperDuperPlugin.js
const fp = require('fp')

const fn = fp((fastify, opts, next) => {
  next()
})

console.log(fn[fp.DISPLAY_NAME_SYMBOL]) // 'mySuperDuperPlugin'

I've also added a unit test.
Regular expressions should work in Node version 6+ too

[mcollina]

Good work! I've left some nits.

[mcollina]

Can you move those two regexps at the top of the file and define them with more clear names?

[mcollina]

can you name this fastify-plugin.display-name, to avoid any possible clash?

[mcollina]

actually this is better as Symbol.for('fastify.display-name'), and we look for it in fastify/fastify#861, and we document it as part as https://github.com/fastify/fastify/blob/master/docs/Plugins.md.

I would not export the symbol from this module then.

[mcollina]

LGTM

[Ethan-Arrowood]

😁 I'll start integrating this with toString

[delvedor]

LGTM!