Add width setting for the file finder (#18682)

This PR adds the ability to adjust the width of the file finder popup. I
found when searching my projects the default width was not always wide
enough and there was no option to change it.

It allows values `small`, `medium` (default), `large`, `xlarge`, and
`full`

Release Notes:

- Added a setting to adjust the width of the file finder modal


Example Setting:
```json
  "file_finder": {
    "modal_width": "medium"
  },
```

Screenshots can be found in the comments below.