It's alt+leftArrow and alt+rightArrow.
Other useful ones which work in native text boxes:
cmd+leftArrow and cmd+rightArrow to move to the start and end of lines in text boxes
cmd+upArrow and cmd+downArrow to move to the very top and very bottom
ctrl+a and ctrl+e to move to the start and end of paragraphs
shift+ any of the above to move the current selection
alt+backspace and cmd+backspace to delete the previous word or to the start of the line
There are more emacs-like keyboard shortcuts in the native text boxes, like ctrl+t to transpose letters, but I don't really use them that much.Ctrl+backspace to delete the previous word in Windows always catches me out, since it works some of the time, but other times is just inserts a control character which I then also have to delete...