Export a LAZYGIT_COLUMNS variable to "pty" tasks on Windows

This makes it possible to pass it to an external diff command that is
used like a pager. An example for this can be seen in the added
documentation in the next commit.

pkg/gui/pty_windows.go

@@ -1,6 +1,7 @@
 package gui
 
 import (
+	"fmt"
 	"os/exec"
 
 	"github.com/jesseduffield/gocui"
@@ -11,5 +12,6 @@ func (gui *Gui) onResize() error {
 }
 
 func (gui *Gui) newPtyTask(view *gocui.View, cmd *exec.Cmd, prefix string) error {
+	cmd.Env = append(cmd.Env, fmt.Sprintf("LAZYGIT_COLUMNS=%d", view.InnerWidth()))
 	return gui.newCmdTask(view, cmd, prefix)
 }