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

crates/vim/src/state.rs

@@ -68,6 +68,7 @@ pub enum Operator {
     Lowercase,
     Uppercase,
     OppositeCase,
+    Digraph { first_char: Option<char> },
     Register,
     RecordRegister,
     ReplayRegister,
@@ -309,6 +310,7 @@ impl Operator {
             Operator::Delete => "d",
             Operator::Yank => "y",
             Operator::Replace => "r",
+            Operator::Digraph { .. } => "^K",
             Operator::FindForward { before: false } => "f",
             Operator::FindForward { before: true } => "t",
             Operator::FindBackward { after: false } => "F",
@@ -342,6 +344,7 @@ impl Operator {
             | Operator::RecordRegister
             | Operator::ReplayRegister
             | Operator::Replace
+            | Operator::Digraph { .. }
             | Operator::ChangeSurrounds { target: Some(_) }
             | Operator::DeleteSurrounds => true,
             Operator::Change