Rework `TextEditor` api

[pokey]

As of #1143, the TextEditor type now has many functions. We'd like to revisit this api, with the following questions:

• Which of these functions belong on the TextEditor?
• Should we move them to the top-level ide type, or split them into components?
• We should enforce that no range is passed in as an argument if the capabilities don't allow them, rather than just relying on SimpleIdeCommandAction to do the right thing. Ideally would do this with type narrowing; barring that could at least add some checks in the VSCode and forthcoming lsp implementations
• Do we want to enforce that some of these functions accept a single range, as we do today?
• Do we want some of these methods to support multiple editors?