smarty: Add a special case for 's at the beginning of the token

[mitya57]

When 's is not preceded by anything, it probably means that it comes after some HTML tag, so it should be converted to a closing quote.

The reference Perl implementation makes the close_class optional and adds a lookahead check for (\s|s\b) when close_class was not matched:

# Single closing quotes:
s {
    ($close_class)?
    '
    (?(1)|          # If $1 captured, then do nothing;
      (?=\s | s\b)  # otherwise, positive lookahead for a whitespace
    )               # char or an 's' at a word ending position. This
                    # is a special case to handle something like:
                    # "<i>Custer</i>'s Last Stand."
} {$1&#8217;}xgi;

Let's copy that behavior by removing closeClass lookbehind check from closingSingleQuotesRegex2.

Fixes #1305.