Scanner.checkPlain

Check if the next token is a plain scalar.

A plain scalar may start with any non-space character except: '-', '?', ':', ',', '[', ']', '{', '}', '#', '&', '*', '!', '|', '>', '\'', '\"', '%', '@', '`'.

It may also start with '-', '?', ':' if it is followed by a non-space character.

Note that we limit the last rule to the block context (except the '-' character) because we want the flow context to be space independent.