### What
- Rename main function to 'main' and add --env flag for environment
modification
- Fix region handling to properly check for null values
### Why
The aws/select-aws-profile script syntax is outdated
```shell
➜ nu
Error: nu::parser::named_as_module
× Can't export command named same as the module.
╭─[~/.config/nushell/nu_scripts/modules/aws/select-aws-profile.nu:12:12]
11 │ # select-aws-profile
12 │ export def select-aws-profile [] {
· ─────────┬────────
· ╰── can't export from module select-aws-profile
13 │ hide AWS_REGION;
╰────
help: Module select-aws-profile can't export command named the same as the module. Either change the module name, or export `main` command.
```
