add wildcard locale support to lang() and locale() methods

this allows the user to test for wildcard territories/languages.

e.g.
```scss
@if(locale(en_)) {
   // matches: en_US, en_GB, en_AU, etc
}

@if(locale(zh_)) {
  // matches: zh_TW, zh_CN, etc
}

@if(locale(_FI)) {
  // matches: fi_FI, sv_FI, etc
}
```