return the full path of 'Microsoft Word' executable if found.
word_exec()executable full path in a character vector of length 1.
Other executable full path:
chrome_exec(),
excel_exec(),
firefox_exec(),
libreoffice_exec(),
node_exec(),
npm_exec(),
pip_exec(),
powerpoint_exec(),
python_exec()
if(exec_available("word"))
message(word_exec())