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/test_data/test_digraph_find.json

@@ -0,0 +1,12 @@
+{"Put":{"state":"ˇHellö world"}}
+{"Key":"f"}
+{"Key":"ctrl-k"}
+{"Key":"o"}
+{"Key":":"}
+{"Get":{"state":"Hellˇö world","mode":"Normal"}}
+{"Put":{"state":"ˇHellö world"}}
+{"Key":"t"}
+{"Key":"ctrl-k"}
+{"Key":"o"}
+{"Key":":"}
+{"Get":{"state":"Helˇlö world","mode":"Normal"}}

crates/vim/test_data/test_digraph_insert_mode.json

@@ -0,0 +1,21 @@
+{"Put":{"state":"Hellˇo"}}
+{"Key":"a"}
+{"Key":"ctrl-k"}
+{"Key":"o"}
+{"Key":":"}
+{"Key":"escape"}
+{"Get":{"state":"Helloˇö","mode":"Normal"}}
+{"Put":{"state":"Hellˇo"}}
+{"Key":"a"}
+{"Key":"ctrl-k"}
+{"Key":":"}
+{"Key":"o"}
+{"Key":"escape"}
+{"Get":{"state":"Helloˇö","mode":"Normal"}}
+{"Put":{"state":"Hellˇo"}}
+{"Key":"i"}
+{"Key":"ctrl-k"}
+{"Key":"o"}
+{"Key":":"}
+{"Key":"escape"}
+{"Get":{"state":"Hellˇöo","mode":"Normal"}}

crates/vim/test_data/test_digraph_keymap_conflict.json

@@ -0,0 +1,7 @@
+{"Put":{"state":"Hellˇo"}}
+{"Key":"a"}
+{"Key":"ctrl-k"}
+{"Key":"s"}
+{"Key":","}
+{"Key":"escape"}
+{"Get":{"state":"Helloˇş","mode":"Normal"}}

crates/vim/test_data/test_digraph_replace.json

@@ -0,0 +1,6 @@
+{"Put":{"state":"Hellˇo"}}
+{"Key":"r"}
+{"Key":"ctrl-k"}
+{"Key":"o"}
+{"Key":":"}
+{"Get":{"state":"Hellˇö","mode":"Normal"}}

crates/vim/test_data/test_digraph_replace_mode.json

@@ -0,0 +1,19 @@
+{"Put":{"state":"ˇHello"}}
+{"Key":"shift-r"}
+{"Key":"ctrl-k"}
+{"Key":"a"}
+{"Key":"'"}
+{"Key":"ctrl-k"}
+{"Key":"e"}
+{"Key":"`"}
+{"Key":"ctrl-k"}
+{"Key":"i"}
+{"Key":":"}
+{"Key":"ctrl-k"}
+{"Key":"o"}
+{"Key":"~"}
+{"Key":"ctrl-k"}
+{"Key":"u"}
+{"Key":"-"}
+{"Key":"escape"}
+{"Get":{"state":"áèïõˇū","mode":"Normal"}}