Add backspace bindings to vi-mode

This was erroneously omitted from the previous commit.

Now backspace in insert mode does backward-delete-char, in default mode
backward-char (i.e. no deleting, just moving). This is consistent with vim.

doc_src/index.hdr.in

@@ -1058,12 +1058,16 @@ Command mode is also known as normal mode.
 - @key{Control, X} copies the current buffer to the system's clipboard, @key{Control, V} inserts the clipboard contents.
 - @key{Control,C} deletes the entire line.
 
+- @key{Backspace} moves the cursor left.
+
 \subsubsection vi-mode-insert Insert mode
 
 - @key{Escape} or @key{Control,C} enters <a href="#vi-mode-command">command mode</a>.
 
 - @key{Control,x} moves the cursor to the end of the line. If an autosuggestion is available, it will be accepted completely.
 
+- @key{Backspace} removes one character to the left.
+
 \subsubsection vi-mode-visual Visual mode
 
 - @cursor_key{&larr;,Left} and @cursor_key{&rarr;,Right} extend the selection backward/forward by one character.

share/functions/fish_vi_key_bindings.fish

@@ -97,8 +97,11 @@ function fish_vi_key_bindings --description 'vi-like key bindings for fish'
 
     bind -k dc delete-char
 
-    bind -k backspace backward-delete-char
-    bind \x7f backward-delete-char
+    # Backspace deletes a char in insert mode, but not in normal/default mode.
+    bind -M insert -k backspace backward-delete-char
+    bind -M default -k backspace backward-char
+    bind -M insert \x7f backward-delete-char
+    bind -M default \x7f backward-char
     bind \e\[3\;2~ backward-delete-char # Mavericks Terminal.app shift-delete
 
     bind dd kill-whole-line