Spec Module (/packages/spec):  Add more specific return types to locale utility functions

Some functions in `locales.ts` could benefit from more precise TypeScript return types. For example:

```
// Current implementation
export const getLocaleCodeDelimiter = (locale: string): string | null => {
 // ...
}
```

```
// Could be improved to
export const getLocaleCodeDelimiter = (locale: string): "-" | "_" | null => {
 // ...
}
```
