This is similar to the IActiveCodeEditor class

Many places  in `extHostLanguageFeatures` were calling `getDocumentData` without checking if the result is undefined. Add an `getDocument` that cannot return undefined values and instead throws an error if there is no document

Fixes #68006

Fixes #68422

Part of #60565

Part of #60565

This reverts commit 80417805.

This reverts commit 2422c987.