Xi-editor: Move editing functions out of editor.rs

[cmyr]

Similar to #1198, we would like to be able to use editing functions independently of an Editor or a runloop.

For more detail, see #1187. For an illustration of how this could be done, see the edit_ops mod in the xi-modal branch. (Note that having this be in a submodule of editor.rs isn't desirable long term).

A goal of this work should be to make it much easier to write simple tests of various edit operations.

[KeeganMyers]

I am a relative beginner to rust and completely new to the code base. Would this be a decent 1st PR opportunity or should I look into another issue? If so I can probably start looking at this over the weekend.

[cmyr]

This is definitely a little bit of a project, but it isn't crazy, and the linked resources will be useful. It would certainly be a good way to become more familiar with the codebase.

[Iamshankhadeep]

I am new to this community and rust, Can I work on this issue?

[nangtrongvuon]

@Iamshankhadeep Sure, feel free to dive into this and ask around on the xi zulipchat if you need help!

[TDecki]

I think this issue can be closed now.

[LiHRaM]

Yep!