functions: add vimscratch

use `vimscratch` to open a scratch vim buffer.

If run in a pipe, it uses `stdin` as input:

```bash
<command> | vimscratch
```

functions/vim.zsh

@@ -0,0 +1,12 @@
+# open vim with a scratch window that can be discarded on exit.
+vimscratch() {
+    local args
+    # if running in a pipe, use stdin (-) as arg.
+    # -t checks if the given FD is a terminal
+    if [ -t 0 ] ; then
+        args="$@"
+    else
+        args="-"
+    fi
+    vim -c "set buftype=nofile" "$args"
+}