Add Vim digraphs (#14887)

Vim digraphs are a way to insert special characters using sequences of
two ASCII characters. I've implemented the feature using a new `Digraph`
operator, following the example of `AddSurrounds`. There are still a few
issues that I'm not sure what the best way to resolve them is.

- To insert `ş`, the user must pause between pressing `ctrl-k` and `s
,`, otherwise it triggers the binding for `ctrl-k s`. Is there a way to
disable `ctrl-k *` bindings while in insert, replace or waiting mode?
- Is there a better way to insert a string at all of the cursors? At the
moment I'm constructing the edits manually.
- The table of default digraphs is a 1.4k line rust expression. Is this
okay as long as it's in its own module?
- I'd like a second opinion on how best to structure the settings.json
entry.
- I have omitted the "meta character" feature as I don't think it makes
sense when editing UTF-8 text.

Release Notes:

- Added support for Vim digraphs.

Resolves #11871

Cargo.lock

@@ -12034,6 +12034,7 @@ dependencies = [
  "indoc",
  "itertools 0.11.0",
  "language",
+ "lazy_static",
  "log",
  "lsp",
  "multi_buffer",