Fix short names of countries in i18n.ts

[chase0213]

Bug, feature request, or proposal:

proposal

What is the expected behavior?

export const I18n = {
  enUS: {
    dayOfWeek: {
      default: [
...

What is the current behavior?

export const I18n = {
  en: {
    dayOfWeek: {
      default: [
...

What are the steps to reproduce?

See https://github.com/chase0213/ng-infinite-calendar/blob/master/src/i18n.ts

What is the use-case or motivation for changing an existing behavior?

en is ambiguous because enUS, enGB and more exist.

Which versions of package, node, npm or other packages are affected?

0.1.4

Is there anything else we should know?

nothing