“Live” previews doesn't follow changes of file system based external refs

With the live preview command `bump preview --live` if the provided definition uses file system based references, then those external references are not “watched” by the CLI.

E.g. `openapi.yaml`
```
paths:
/v1/version:
$ref: 'v1/version.yaml'
```
With a Path Item object of the `/v1:version` endpoint in the external file `v1/version.yaml`

## Steps to reproduce

- Launch the live preview command `bump preview --live --open openapi.yaml`
- Change the `v1/version.yaml` file
- Nothing happens

## What should happen

- Launch the live preview command `bump preview --live --open openapi.yaml`
- Change the `v1/version.yaml` file
- The preview should be updated with the latest file system changes