Add shift-backspace keybind alias for backspace (helix-editor#4937)

When the Kitty Keyboard Protocol is enabled, S-backspace is
distinguished from backspace with no modifiers. This is awkward when
typing because it's very easy to accidentally hold shift and press
backspace temporarily when typing capital letters.

Kakoune (which is also a Kitty Keyboard Protocol application) treats
S-backspace as backspace too:
https://github.com/mawww/kakoune/blob/3150e9b3cd8e61d9bc68245d67822614d4376cf4/src/input_handler.cc#L1275

Author: lesleyrs

book/src/keymap.md

@@ -352,7 +352,7 @@ experience.
 | `Alt-d`, `Alt-Delete`                       | Delete next word            | `delete_word_forward`    |
 | `Ctrl-u`                                    | Delete to start of line     | `kill_to_line_start`     |
 | `Ctrl-k`                                    | Delete to end of line       | `kill_to_line_end`       |
-| `Ctrl-h`, `Backspace`                       | Delete previous char        | `delete_char_backward`   |
+| `Ctrl-h`, `Backspace`, `Shift-Backspace`    | Delete previous char        | `delete_char_backward`   |
 | `Ctrl-d`, `Delete`                          | Delete next char            | `delete_char_forward`    |
 | `Ctrl-j`, `Enter`                           | Insert new line             | `insert_newline`         |
 
@@ -433,7 +433,7 @@ Keys to use within prompt, Remapping currently not supported.
 | `Alt-d`, `Alt-Delete`, `Ctrl-Delete`        | Delete next word                                                        |
 | `Ctrl-u`                                    | Delete to start of line                                                 |
 | `Ctrl-k`                                    | Delete to end of line                                                   |
-| `Backspace`, `Ctrl-h`                       | Delete previous char                                                    |
+| `Backspace`, `Ctrl-h`, `Shift-Backspace`    | Delete previous char                                                    |
 | `Delete`, `Ctrl-d`                          | Delete next char                                                        |
 | `Ctrl-s`                                    | Insert a word under doc cursor, may be changed to Ctrl-r Ctrl-w later   |
 | `Ctrl-p`, `Up`                              | Select previous history                                                 |

helix-term/src/keymap/default.rs

@@ -363,7 +363,7 @@ pub fn default() -> HashMap<Mode, Keymap> {
         "A-d" | "A-del" => delete_word_forward,
         "C-u" => kill_to_line_start,
         "C-k" => kill_to_line_end,
-        "C-h" | "backspace" => delete_char_backward,
+        "C-h" | "backspace" | "S-backspace" => delete_char_backward,
         "C-d" | "del" => delete_char_forward,
         "C-j" | "ret" => insert_newline,
         "tab" => insert_tab,

helix-term/src/ui/prompt.rs

@@ -516,7 +516,7 @@ impl Component for Prompt {
             alt!('d') | alt!(Delete) | ctrl!(Delete) => self.delete_word_forwards(cx.editor),
             ctrl!('k') => self.kill_to_end_of_line(cx.editor),
             ctrl!('u') => self.kill_to_start_of_line(cx.editor),
-            ctrl!('h') | key!(Backspace) => {
+            ctrl!('h') | key!(Backspace) | shift!(Backspace) => {
                 self.delete_char_backwards(cx.editor);
                 (self.callback_fn)(cx, &self.line, PromptEvent::Update);
             }