country flag emoji are not represented homogenously

### Steps to reproduce

1. Open the Element Web client (I'd test this on a desktop client but I've only a chromebook)
2. Select a country (you must know that country's two-letter abbreviation)
3. Attempt to type the emoji for that country's flag

### What happened?

### What did you expect?
All country flags should be represented the same way. In Discord, for example, country flags follow the format `:flag_xx:`, where `xx` is the two-letter abbreviation for the country in question.

### What happened?
Different countries follow different patterns.
There are, as far as I can tell, two distinct patterns:
Pattern A is `:xx:`, where `xx` is the country's two-letter abbreviation.
Pattern B is `:flag-xx:`, where `xx` is the country's two-letter abbreviation.

Below is an incomplete list of which countries use which patterns.

Countries that use Pattern A:
- United Kingdom
- France
- Spain
- Germany
- China
- Japan
- Russia
- South Korea

Countries that use Pattern B:
- Portugal
- Canada
- Mexico
- North Korea
- Australia

### Operating system

_No response_

### Browser information

_No response_

### URL for webapp

_No response_

### Homeserver

_No response_

### Have you submitted a rageshake?

No