Fixes #91670

- Move model type logic out of `CustomEditorInput` and into two different implementations of `ICustomEditorModel`
- Add `CustomTextEditorModel` that owns a proper model reference to a text document. This should ensure the text document is disposed of if there are no more references to it
- Move most of the previous `CustomEditorModel` into `mainThreadWebviews` directly. This removes some of the indirection that was previously required (such as using `waitUntil`)