add `insert_character_interactive` command

this command inserts a character of the user's choice underneath the current cursor, without exiting normal mode. multiple cursors are handled correctly.

i chose `C-y` for the default mapping because it doesn't conflict with anything else. i would have preferred `C-i` (i for insert), but that conflicts with `jump_forward`; and additionally most terminals do not distinguish `C-i` from `tab`, so it's not a good choice for the default.

this command does not have precedence in vim or kakoune to my knowledge, but at least vim allows replicating it with remap command (https://superuser.com/questions/581572/insert-single-character-in-vim), while helix currently does not, since there's no way to say "a character entered by the user".

helix-term/src/commands.rs

@@ -397,6 +397,7 @@ impl MappableCommand {
         smart_tab, "Insert tab if all cursors have all whitespace to their left; otherwise, run a separate command.",
         insert_tab, "Insert tab char",
         insert_newline, "Insert newline char",
+        insert_char_interactive, "Insert an interactively-chosen char",
         delete_char_backward, "Delete previous char",
         delete_char_forward, "Delete next char",
         delete_word_backward, "Delete previous word",
@@ -3727,7 +3728,7 @@ fn hunk_range(hunk: Hunk, text: RopeSlice) -> Range {
 }
 
 pub mod insert {
-    use crate::events::PostInsertChar;
+    use crate::{events::PostInsertChar, key};
 
     use super::*;
     pub type Hook = fn(&Rope, &Selection, char) -> Option<Transaction>;
@@ -3802,11 +3803,15 @@ pub mod insert {
     }
 
     pub fn insert_tab(cx: &mut Context) {
+        insert_tab_impl(cx, 1)
+    }
+
+    fn insert_tab_impl(cx: &mut Context, count: usize) {
         let (view, doc) = current!(cx.editor);
         // TODO: round out to nearest indentation level (for example a line with 3 spaces should
         // indent by one to reach 4 spaces).
 
-        let indent = Tendril::from(doc.indent_style.as_str());
+        let indent = Tendril::from(doc.indent_style.as_str().repeat(count));
         let transaction = Transaction::insert(
             doc.text(),
             &doc.selection(view.id).clone().cursors(doc.text().slice(..)),
@@ -3815,6 +3820,34 @@ pub mod insert {
         doc.apply(&transaction, view.id);
     }
 
+    pub fn insert_char_interactive(cx: &mut Context) {
+        let count = cx.count();
+
+        // need to wait for next key
+        cx.on_next_key(move |cx, event| {
+            match event {
+                KeyEvent {
+                    code: KeyCode::Char(ch),
+                    ..
+                } => {
+                    for _ in 0..count {
+                        insert::insert_char(cx, ch)
+                    }
+                }
+                key!(Enter) => {
+                    if count != 1 {
+                        cx.editor
+                            .set_error("inserting multiple newlines not yet supported");
+                        return;
+                    }
+                    insert_newline(cx)
+                }
+                key!(Tab) => insert_tab_impl(cx, count),
+                _ => (),
+            };
+        });
+    }
+
     pub fn insert_newline(cx: &mut Context) {
         let (view, doc) = current_ref!(cx.editor);
         let text = doc.text().slice(..);