editor/language: hoist out non-generic parts of edit functions. (#3130)

This reduces LLVM IR size of editor (that's one of the heaviest crates
to build) by almost 5%.

LLVM IR size of `editor` before this PR: 3280386
LLVM IR size with `editor::edit` changed: 3227092
LLVM IR size with `editor::edit` and `language::edit` changed: 3146807

Release Notes:
- N/A