feat: [hx] r to replace with a character

If no selection, the character under the cursor is replaced, if
selection, each character is replaced by the typed character.

src/keybind/builtin/helix.json

@@ -379,6 +379,7 @@
             ["N", "extend_search_next"],
             ["*", "extend_search_prev"],
 
+            ["r", "replace"],
             ["P", "paste_clipboard_before"],
             ["R", ["replace_selections_with_clipboard"], ["enter_mode", "normal"]],
             ["p", "paste_after"],
@@ -546,6 +547,17 @@
             ["8", "add_integer_argument_digit", 8],
             ["9", "add_integer_argument_digit", 9]
         ]
+    },
+    "mini/replace": {
+        "press": [
+            ["ctrl+g", "mini_mode_cancel"],
+            ["ctrl+c", "mini_mode_cancel"],
+            ["ctrl+l", "scroll_view_center_cycle"],
+            ["tab", "mini_mode_insert_bytes", "\t"],
+            ["enter", "mini_mode_insert_bytes", "\n"],
+            ["escape", "mini_mode_cancel"],
+            ["backspace", "mini_mode_cancel"]
+        ]
     }, 
     "home": {
         "on_match_failure": "ignore",