Fix "fm_open_in_explorer" in sidebar causes an exception

[jfcherng]

In 74be57b, visible_on_platforms should be a optional arg. Otherwise, it's a BC breaking change for where visible_on_platforms is not specified such as sidebar or custom keybindings.

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We need a new release to fix this since currently fm_open_in_explorer causes an exception right after you right click on the sidebar.

Traceback (most recent call last):
  File "D:\_Download\ST-SM\sublime_text_build_4129_x64\Lib\python38\sublime_plugin.py", line 1380, in is_visible_
    ret = self.is_visible()
TypeError: is_visible() missing 1 required positional argument: 'visible_on_platforms'

[math2001]

Thanks 👍