Add comment lines to keymap wrapper files

[tshino]

To make it easy to understand the keymap wrapper, which is required for users to copy and paste into keybindings.json manually, this PR adds comment lines to autogenerated keymap wrappers to describe itself.

[
	// Keymap wrapper for Vz Keymap v0.18.0
	// (required by Keyboard Macro Beta)
	{ "key": "ctrl+s", "command": "",
		"when": "kb-macro.recording && editorFocus" },
...