feature request: dynamic defaults

in some cases a default value for an argument might be from an env. variable (or otherwise computed), in which case using a `.default(process.env.FOO)` would a) result in bad ux for the user in `--help` (they wouldn't know where the value comes from), and b) might be expensive

currently the workaround is to keep the argument as optional, put the default value description in the `.desc()` and use `??` in handler/transform function, which adds a bit of boilerplate

i propose an overload for the `.default()` method that would accept a function and optionally a textual description of what happens there:

```ts
const foo = command({
name: 'foo',
options: {
foo: string().default(() => process.env.FOO, 'process.env.FOO'),
bar: number().default(() => 2 ** 32),
}
})
```