Add Helix editor to documentation.

README.md

@@ -174,6 +174,7 @@ or dozens of notebooks containing thousands of notes, bookmarks, and other items
 - A text editor with command line support, such as:
   - [Vim](https://en.wikipedia.org/wiki/Vim_\(text_editor\)),
   - [Emacs](https://en.wikipedia.org/wiki/Emacs),
+  - [Helix](https://helix-editor.com/),
   - [Visual Studio Code](https://code.visualstudio.com/),
   - [Sublime Text](https://www.sublimetext.com/),
   - [micro](https://github.com/zyedidia/micro),
@@ -4986,6 +4987,7 @@ the setting name or number to [`nb set`](#settings):
          atom
          code
          emacs
+         hx
          macdown
          mate
          micro
@@ -8019,6 +8021,7 @@ Alias:
          atom
          code
          emacs
+         hx
          macdown
          mate
          micro

docs/index.markdown

@@ -180,6 +180,7 @@ or dozens of notebooks containing thousands of notes, bookmarks, and other items
 - A text editor with command line support, such as:
   - [Vim](https://en.wikipedia.org/wiki/Vim_\(text_editor\)),
   - [Emacs](https://en.wikipedia.org/wiki/Emacs),
+  - [Helix](https://helix-editor.com/),
   - [Visual Studio Code](https://code.visualstudio.com/),
   - [Sublime Text](https://www.sublimetext.com/),
   - [micro](https://github.com/zyedidia/micro),
@@ -2277,7 +2278,7 @@ See [`bookmark help`](#bookmark-help) for more information.
   </sup>
 </p>
 
-Use [`nb todo`](#todo) (shortcut: [`nb t`](#todo))
+Use [`nb todo`](#todo) (shortcut: [`nb to`](#todo))
 to create, list, and update todos.
 `nb` todos are [structured Markdown documents](#nb-markdown-todo-file-format)
 referencing a single primary todo,
@@ -4992,6 +4993,7 @@ the setting name or number to [`nb set`](#settings):
          atom
          code
          emacs
+         hx
          macdown
          mate
          micro
@@ -8025,6 +8027,7 @@ Alias:
          atom
          code
          emacs
+         hx
          macdown
          mate
          micro

nb

@@ -646,6 +646,7 @@ __set_editor() {
   local _editors=(
     code
     subl
+    hx
     micro
     mate
     macdown
@@ -695,6 +696,10 @@ More information about setting \$EDITOR:
     https://en.wikipedia.org/wiki/Emacs
 - TextMate
     https://macromates.com
+- Helix
+    https://helix-editor.com
+- micro
+    https://github.com/zyedidia/micro
 - nano
     https://en.wikipedia.org/wiki/GNU_nano
 - or many of these:
@@ -24320,7 +24325,7 @@ $(
 
 $(
   local __editor=
-  for   __editor in atom code emacs macdown mate micro nano pico subl vi vim
+  for   __editor in atom code emacs hx macdown mate micro nano pico subl vi vim
   do
     if _command_exists "${__editor}"
     then
@@ -24661,6 +24666,7 @@ HEREDOC
               -e '^code'                      \
               -e '^emacs'                     \
               -e '^gvim'                      \
+              -e '^hx'                        \
               -e '^macdown'                   \
               -e '^mate'                      \
               -e '^mvim'                      \