Moves this forward by one character offset.
Note that images embedded in the buffer occupy 1 character slot, so forward_char may actually move onto an image instead of a character, if you have images in your buffer. If this is the end iterator or one character before it, this will now point at the end iterator, and forward_char returns false for convenience when writing loops.
| this |
an iterator |
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whether this moved and is dereferenceable |