feat: add showCountryCode parameter to PhoneInputFormatter

[bernardoveras]

Summary

• Added showCountryCode parameter (default: true) to PhoneInputFormatter, formatAsPhoneNumber, and the isFilled / unmasked getters.
• When false, the formatted output displays only the local portion of the number (e.g. (11) 99999-9999) while unmasked always returns the full international number (e.g. +5511999999999).
• Extracted duplicated strip-and-trim logic from _applyMask and formatAsPhoneNumber into a single private _resolveCountryCodeDisplay helper.
• Fixed a latent negative-offset assertion crash in formatEditUpdate by clamping selectionEnd — this could trigger when showCountryCode: false produces a masked value shorter than the input and the cursor is positioned early in the text.

Usage

// Default — country code visible (backward compatible)
PhoneInputFormatter(showCountryCode: true)
// input: '+5511999999999' → masked:   '+55 (11) 99999-9999'
//                        → unmasked: '+5511999999999'

// Hide country code
PhoneInputFormatter(showCountryCode: false)
// input: '+5511999999999' → masked:   '(11) 99999-9999'
//                        → unmasked: '+5511999999999'  ← full number preserved

// Top-level helper
formatAsPhoneNumber('+5511999999999', showCountryCode: false);
// → '(11) 99999-9999'

// Combined with defaultCountryCode
PhoneInputFormatter(defaultCountryCode: 'BR', showCountryCode: false)
// input: '11999999999' → masked:   '(11) 99999-9999'
//                      → unmasked: '+5511999999999'

Breaking changes

None. showCountryCode defaults to true, preserving all existing behaviour exactly.

Test plan

• flutter test test/flutter_multi_formatter_test.dart — all existing tests pass; 9 new showCountryCode tests added covering BR, US, partial input, unmasked getter, formatAsPhoneNumber, isPhoneValid, and the defaultCountryCode combination.
• flutter analyze lib/formatters/phone_input_formatter.dart — no issues.
• Manual: attach PhoneInputFormatter(showCountryCode: false) to a TextField, enter a Brazilian number (+5511999999999) and confirm display shows (11) 99999-9999 while formatter.unmasked returns +5511999999999.