formatters: Add formatters-custom for overrides

[creotutar]

Previously, in order for HHs to override formatters, they would need to
theme override the formatters.js class /and/ reference any new
formatters from some new javascript class. We standardize this process
with the formatters-custom.js file. This new file will be moved to the
top-level static/js where HHs can change it without overriding files.

This file allows a HH to override current formatters as well as define
new ones.

J=SLAP-810
TEST=manual

Tested that with a formatters-custom in js/static of a HH theme
repository, you can write

export function address(profile) {
  return `111 Yext Street`;
}
export function newFunction(profile) {
  return 'helllllooooo';
}

and you can expect, after a jambo build that the addresses all
return the 111 Yext Street string and you can go into the command line
and write HitchhikerJS.Formatteres.newFunction() and get the hello
string.

[cea2aj]

🎃 💀 👻

[tmeyer2115]

LGTM! Once tree shaking is working, we'll want to test that if I have a custom override of a formatter, the built-in formatter, and it's dependencies, are shaken out. As a contrived example, if I overwrote nationalizedPhoneDisplay without libphonenumber, then libphonenumber would not be part of bundle.js.