Firefox: Polyfill for selection.type for visual mode.
Firefox: Polyfill for selection.type for visual mode.

@mrmr1993... This puts the logic of how we choose the selection type in
one place; so, if you have a better idea of how to determine the
selection type, then we just change it here.  Once.

mrmr1993/vimium,decaffeinate-examples/vimium,mrmr1993/vimium,gdh1995/vimium,mrmr1993/vimium,gdh1995/vimium,smblott-github/vimium,decaffeinate-examples/vimium,poacher2k/vimium,philc/vimium,gdh1995/vimium,philc/vimium,philc/vimium,smblott-github/vimium,poacher2k/vimium,smblott-github/vimium,poacher2k/vimium,decaffeinate-examples/vimium