This PR removes the dependents of the `language_models` crate.

The following types have been moved from `language_models` to
`language_model` to facilitate this:

- `LlmApiToken`
- `RefreshLlmTokenListener`
- `MaxMonthlySpendReachedError`
- `PaymentRequiredError`

With this change only `zed` now depends on `language_models`.

Release Notes:

- N/A