Add optional 'version' field to the 'prelude`

So it will look like this:

```toml
[prelude]
package = "relude"
module = "Relude"
version = "0.6.0.0"
```

And in the `.cabal` file you'll have `relude ^>= 0.6.0.0`

**Open questions**

1. Should we do any validation, like, check that all components are only numbers?
2. What to do with `stack`? Always add to `extra-deps` or just ignore it?

**Alternatives**

Allow arbitrary text in the `version`, so users can write something like:

```haskell
version = "^>= 0.6.0.0"
```

or

```haskell
version = ">= 0.5.0 && < 0.7"
```

Locate the existing `prelude` handling and the entry points that generate `.cabal` and `stack` configuration. Resolve the stated validation and `stack` behavior questions, then verify that a `version` value produces the intended Cabal constraint and that the chosen behavior is covered by tests.

Written by the indexing model from the issue text.