Reads a single character from the read stream and outputs it. If the read stream was opened for text reading, then the character is output as a word. If the read stream was opened for binary reading, then the character is output as a number.
If the read stream is a file and the end of file is reached, READCHAR outputs the empty list (not the empty word).
If the read stream is the Commander, then the "Input Mode" dialog prompts the user for a line of text and READCHAR outputs the first character entered by the user. Successive calls to READCHAR read the rest of the line until there are no more characters, in which case the "Input Mode" dialog is shown again.
Backslash, vertical bar, and tilde characters have no special meaning in this context.
SHOW READCHAR Hello how are you?H