Create DigitalOcean region module

In the DigitalOcean template ([do-linux](https://github.com/coder/coder/tree/main/examples/templates/do-linux)), we use a `coder_parameter` for region selection ([main.tf#L170-L229](https://github.com/coder/coder/blob/45d9274aca31860e6da8cac0e300590f683e29eb/examples/templates/do-linux/main.tf#L170-L229)).

Let's make this a module.