(<SC_CHARACTER>): Don't worry about any backslash

(<SC_CHARACTER>): Don't worry about any backslash
escapes other than \\ and \'; this simplifies the code.
(<SC_STRING>): Likewise, for \\ and \".
(<SC_COMMENT,SC_LINE_COMMENT,SC_STRING,SC_CHARACTER,SC_BRACED_CODE,
SC_PROLOGUE,SC_EPILOGUE>): Escape $ and @, too.
Use new escapes @{ and @} for [ and ].