update: libxlsxwriter 1.2.4 with upstream directory structure

Restructure vendored libxlsxwriter source to mirror the upstream distribution layout under src/libxlsxwriter/: src/libxlsxwriter/src/ (was src/libxlsxwriter/) src/libxlsxwriter/include/ (was src/include/) src/libxlsxwriter/third_party/ (was src/md5/, src/minizip/, src/tmpfileplus/) This makes future updates a simple copy of the src/, include/, and third_party/ directories from the upstream tarball. Update src/Makevars with the new paths and add src/libxlsxwriter/**/Makefile* to .gitignore so upstream Makefiles are not tracked (they trigger R CMD check warnings about GNU extensions and are not used by the R build).