Embedded `Morphs` in `Text` should behave correctly with and without `Document`

Nobody has claimed this yet.

Right now, we force `Document`s on `Text` with embedded `Morphs`. It would be nice to have them work without `Document`s as well.

This spans multiple different problem spaces:

1. [ ] It should be possible to add lines either via drag-and-drop in interactive mode and programmatically by including them in `textAndAttributes`. The second scenario should work with document and without.
2. [ ] Regardless of whether a Text has a document or not, its submorph-array should always contain its embedded morphs. They should also be correctly referenced in the embeddedMorphMap of the text.
3. [ ] When a morph A is embedded into a text, calling A.owner should return the text. This should work with or without document.
4. [ ] When a Morph gets removed from a text (either via drag-and-drop in interactive mode) or because the textAndAttributes of a Text were changed, the submorphs array of the text, its embeddedMorphMap and the owner of the previously embedded morph should be in a consistent state.
5. [ ] Regardless of whether a text is backed by a document or not, embedded morphs should update their positions correctly.

Start by locating the Text implementation and the code handling textAndAttributes, embeddedMorphMap, submorphs, and owner relationships. Trace embedded morph insertion, removal, and position updates with and without a Document, then verify that all five listed behaviors remain consistent for both paths.

Written by the indexing model from the issue text.