Emoji names sometimes have dashes and sometimes have underscores

### Your use case

#### What would you like to do?
Make all emoji names have the same format

Example: At the moment, the Australia flag (`:flag-au:`) has dashes but the majority of emoji names use underscores instead, This is also the case on other chat apps such as Discord.
#### Why would you like to do it?
Because it would make more sense and be less confusing to remember the names
#### How would you like to achieve it?
Change the flag `-` in the emoji names to `_`

### Have you considered any alternatives?

_No response_

### Additional context

_No response_

Search the repository for `flag-au` and the emoji-name definitions; start with the data or lookup entry point that contains the dash-form names. Check how existing tests cover emoji aliases, then confirm that all affected flag names use underscores consistently.

Written by the indexing model from the issue text.