There is no shortcut for it by default. As a hack, you can create
~/Library/KeyBindings/ and save a property list like this as
"~m" = (moveToBeginningOfParagraph:, insertText:, " ", moveLeft:, selectWord:,
moveRight:, setMark:, moveToBeginningOfParagraph:, deleteForward:,
Then quit and reopen applications to apply the changes.
~m is option-m, which is bound to
back-to-indentation in Emacs.
I copied the macro from https://github.com/ttscoff/KeyBindings/blob/master/DefaultKeyBinding.dict.
moveToBeginningOfParagraph:, selectWord:, moveRight: would not work with lines that don't start with spaces or tabs.
moveToBeginningOfParagraph:,moveWordRight:, moveWordLeft: would not work with some lines where the first non-whitespace character is a special character.
DefaultKeyBinding.dict does not work with some applications like Xcode or Firefox.
See http://www.hcs.harvard.edu/~jrus/site/cocoa-text.html or http://osxnotes.net/keybindings.html for more information.