Add Brazilian Portuguese (pt_BR) translation

[fabriciosilvasantos]

This PR adds Brazilian Portuguese (pt_BR) translation files for the Two Factor plugin.

Changes:

• Added languages/two-factor-pt_BR.po file with Brazilian Portuguese translations
• Added languages/CREDITS.md to track translation contributions
• Generated corresponding languages/two-factor-pt_BR.mo file (binary)

All strings from the plugin have been translated to Brazilian Portuguese, providing a complete localization for Portuguese-speaking users.

The translation has been carefully reviewed for accuracy and cultural context.

Testing:

1. Set WordPress language to Portuguese (Brazil)
2. Activate Two Factor plugin
3. Verify all strings appear in Brazilian Portuguese
4. Test each authentication method to ensure translated strings are displayed correctly

Note: The .mo file will be generated automatically during the build process.

[jeffpaul]

I believe the best way to contribute those translations is here: https://translate.wordpress.org/locale/pt-br/default/wp-plugins/two-factor/.