Make locale use more consistent with the Java Locale API

[abeshenov]

The use of locales as suggested by README.md seems to be inconsistent with the Java Locale API. Namely, it gives as an example

new Faker(new Locale("en-us")).address().zipCodeByState("CA"));

Here Locale("en-us") is an incorrect locale with language en-us. This is not the same as Locale.US, which is Locale("en", "US"), i.e. language en and country US.

It seems like Faker normalizes the incorrect locales:

var locale = new Locale("en-us");  // Incorrect locale
var faker = new Faker(locale);

faker.getLocale();  // The correct Locale("en", "US")

This behavior is not reflected in the javadoc. Probably the documentation in README.md could give as an example one of these two:

new Faker(Locale.US).address().zipCodeByState("CA");
// OR
new Faker(new Locale("en", "US")).address().zipCodeByState("CA");

and the list of supported locales could follow the same format as used by Locale::toString, with underscores _ instead of hyphens -:

- ar
- bg
- ca
- ca_CAT
- cs
- da_DK
. . . . .

Another inconsistency with the Java Locale API that I noticed is that java.util.Locale uses ISO 3166 alpha-2 for country codes, and datafaker doesn't. Not everything supported by datafaker can be mapped to ISO 3166 alpha-2, but probably datafaker could recognize ISO 3166 alpha-2 codes whenever possible:

var fakerNp = new Faker(new Locale("en", "NP"));
var fakerNep = new Faker(new Locale("en", "NEP"));

fakerNp.getLocale().getDisplayCountry();  // "Nepal"
fakerNep.getLocale().getDisplayCountry();  // "NEP"

fakerNp.name().fullName();  // "Clora Douglas"
fakerNep.name().fullName();  // "Laxmi Basynat"

As you see, the Java API is not aware of NEP while datafaker is not aware of NE. The same issue arises for IN and IND.