chore(web): Additional special key images - ZWJ, ZWSP, RTL/LTR markers

@rmlockwood, yes, that's a good point -- not sure how we missed that. There may be others to consider as well -- maybe ZWSP? Do you have any suggestions on what ZWJ and ZWSP could look like?

_Originally posted by @mcdurdin in https://github.com/keymanapp/keyman/issues/3878#issuecomment-830360846_

@mcdurdin, ZWSP isn’t relevant for our Arabic script languages so no suggestions. RTL & LTR – right to left marker and left to right marker are somewhat relevant, more on desktop keyboards. I don’t have image suggestions for these. As far as ZWJ, I haven’t found any keyboards on iOS and Android that show it. My only suggestion would be to used a similar image to the ZWNJ on Android, just with the arrows pointing in. Maybe even use this image on iOS.

_Originally posted by @rmlockwood in https://github.com/keymanapp/keyman/pull/3878#issuecomment-831148317_