How to configure alias options?

Re the discussion [here](https://github.com/coder/envbuilder/issues/166#issuecomment-2100971392), it's common to want multiple envs/flags to configure the same underlying option value esp. when refactoring the command line interface.

@mafredri proposes the concept of an `OptionAlias` that embeds within each Option like so:

```go
type OptionAlias struct {
Flag string `json:"flag,omitempty"`
FlagShorthand string `json:"flag_shorthand,omitempty"`
Env string `json:"env,omitempty"`
Deprecated string `json:"deprecated,omitempty"`
}

// Option is a configuration option for a CLI application.
type Option struct {
Aliases []OptionAlias `json:"aliases,omitempty"`
// ...
}

// Usage:
{
// ...
Env: "CODER_FOOBAR",
Alias: []serpent.OptionAlias{
{Env: "FOOBAR", Deprecated: "Use CODER_FOOBAR instead."},
},
},
```

I think the extra level of nesting on all options is unjustified when in the vast majority of options there is only one "alias".