Optional method to export dictionary to JSON using Plover's "save as" feature?

[user202729]

It may be useful to allow exporting a Python dictionary to a JSON file.

I think a possible implementation is something like:

		def _default_items():
			return ()
			raise ValueError("Dictionary does not support exporting!")
		items = mod.get('items', _default_items)
		if not isinstance(items, typing.Callable):
			raise ValueError('invalid `items\' function: %s\n' % items)
		self._items = items

then the Python file, if it wants, can define a items function that returns an iterable of (stroke, value): (str, str) objects.

Motivation: my library https://github.com/user202729/plover-python-dictionary-lib/ supports exporting to JSON, but it would be better to allow doing that inside Plover's GUI itself, so user who are not familiar with the CLI would find it easier.