a simple mode example

How-to-define-custom-mode.md

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+```
+# define some command
+# (define some key to a mode does not exist will create it automatically.)
+bind --mode=my-mode c js alert('hello world!')
+bind --mode=my-mode U fillcmdline undo
+
+# inherit other mode's keymap if you want
+# the mode map name is `${mode_name}maps`
+set my-modemaps.🕷🕷INHERITS🕷🕷 nmaps
+
+# define how to enter and exit mode
+bind gm mode my-mode
+bind --mode=my-mode <Escape> mode normal
+
+# if you want to enter/exit from ex/hint/visual/insert/input,
+# you should take a look of their binding for <Escape>.
+# the `mode` command will not handle these things for you.
+```