[RDY] Extract line_consumption.c from misc1.c

This pull request extracts some functions that are concerned with physical line consumption of a buffer on screen from `misc1.c` and puts it in a file called `screenline.c`. (See #209)
Then I think it's better we wait for #941, as I'd really like to avoid anymore files relying on `vim.h` in a `.h` file (so as to not regress on #918).