package.json:contributes/configuration should have namespaced properties

Our configuration contribution of the extension, which holds extension property settings, doesn't namespace the property names, which seems [to go against recommendations](https://code.visualstudio.com/api/references/contribution-points#contributes.configuration). At runtime all settings are stored in a shared property map, so we could be accidentally clobbering properties by not having a namespace.

We can also load only our settings via:
```diff
- const defaultConf = vscode.workspace.getConfiguration()
+ const defaultConf = vscode.workspace.getConfiguration('daffodil')
```

At the same time, we seem to use the pseudo-namespace `dfdl` in places, and perhaps we should use the more readable `daffodil`. But also, the extension itself uses the id `apache-daffodil-vscode`, so perhaps that should be the namespace we use in all configuration.

Start with package.json's contributes/configuration entry and search the extension for getConfiguration, including the shown defaultConf call and uses of the dfdl or daffodil pseudo-namespaces. Determine which namespace should be standard and identify every affected setting reference. Done means the contribution names and runtime lookups consistently use the agreed namespace without changing unrelated settings.

Written by the indexing model from the issue text.