FR: Standardise the `exit-node list --filter` options

### What are you trying to do?

The `--filter` option (`exit-node list --filter`) is a bit messy! Sometimes you have to write the ISO country code (e.g., UK, etc.), other times the full name of the country (e.g., Italy), and other times the 3-letter country code (e.g., USA, etc.). Why don't you standardize on a 2-letter country code?

### How should we solve this?

**_Standardize the `--filter` option in the `tailscale exit-node list` to accept only one type of value_**

### What is the impact of not solving this?

creates confusion in scripts

### Anything else?

_No response_