Option to exclude country selector chip from focus

[hbatalhaStch]

Gives a better UX on mobile forms whose fields have textInputAction: TextInputAction.next if the country selector chip is excluded from focus. This PR add the property skipFocusOnCountryNavigator(which defaults to false) to the PhoneFormField to control that.

[cedvdb]

Could you explain the scenarios where we would want it to be focusable vs not focusable ? Is making it never focusable an acceptable option ?

[hbatalhaStch]

In the gif below we have a situation where it is not focusable, as you can you see, you click the keyboard next button and the next field gain focus. And that happens after you finish writing the email and click next, you will immediately be prompted to enter the phone number and in a smooth way.

In the gif below we have a situation where it is focusable, after you finish writing the email and click next, the keyboard will go down because the focus goes to the navigator chip which you can do nothing when you are on mobile and you would have to go and click on the phone field to enter the number to proceed, which in my opinion is not a good ux because it breaks the pattern the user was on up until then.

[cedvdb]

Let's make it unfoccussable always without giving the option to change it. This is to not add unnecessary parameters that make it to the public API.

[hbatalhaStch]

Let's make it unfoccussable always without giving the option to change it. This is to not add unnecessary parameters that make it to the public API.

Done