patch 9.2.0140: file reading performance can be improved

patch 9.2.0140: file reading performance can be improved

Problem:  Reading large files is slow because UTF-8 validation and
          newline scanning are performed byte-by-byte. Initial file
          loading also triggers listener and channel processing.
Solution: Use memchr() for SIMD-optimized newline scanning, implement
          word-at-a-time ASCII skipping during UTF-8 validation using a
          bitmask, skip listener/netbeans/channel notifications
          when the ML_APPEND_NEW flag is set during readfile()
          (Yasuhiro Matsumoto).

closes: #19612

Co-authored-by: NRK <nrk@disroot.org>
Signed-off-by: Yasuhiro Matsumoto <mattn.jp@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>