Work around Linux keycodes by mapping ^_ to ^7

On Linux terminal, keycodes for `^/`, `^_`, and `^7` are the same. The
parsed key depends on the library that handles the input. In case of
Zee, it uses Zi framework for terminal based applications, which uses
crossterm library to get key events.

For more details, see crossterm issues:

- crossterm-rs/crossterm#540
- crossterm-rs/crossterm#685

To check if it reproduces on your machine, you can try

```
showkey --ascii
```

- Crossterm - https://github.com/crossterm-rs/crossterm
- Zi - https://github.com/mcobzarenco/zi

zee/src/components/buffer/mod.rs

@@ -481,8 +481,22 @@ impl Component for Buffer {
             .command("undo", |this: &Self| {
                 this.properties.cursor.undo();
             })
-            .with([Ctrl('_')])
             .with([Ctrl('z')])
+            // On Linux terminal, keycodes for `^/`, `^_`, and `^7` are the same. The
+            // parsed key depends on the library that handles the input. In case of Zee,
+            // it uses Zi framework for terminal based applications, which uses
+            // crossterm library to get key events.
+            //
+            // For more details, see crossterm issues:
+            // - https://github.com/crossterm-rs/crossterm/issues/540
+            // - https://github.com/crossterm-rs/crossterm/issues/685
+            //
+            // To check if it reproduces on your machine, you can try
+            // ```shell
+            // showkey --ascii
+            // ```
+            .with([Ctrl('7')])
+            .with([Ctrl('_')])
             .with([Ctrl('/')]);
 
         // Redo