vim-patch:8.2.1741: pathshorten() only supports using one character

Problem:    pathshorten() only supports using one character.
Solution:   Add an argument to control the length. (closes vim/vim#7006)
6a33ef0deb

Cherry-pick a line in test from patch 8.2.0634.
Use Nvim's config paths in docs.

shorten_dir() returning a pointer looks a bit confusing here, as it is
actually the same pointer passed to it, and it doesn't really reduce
much code, so change it back to void.

Assigning rettv->vval.v_string = NULL is not needed if a pointer is
within 64 bits. While this is usually the case, I'm not sure if it can
be taken for granted.

src/nvim/eval.lua

@@ -262,7 +262,7 @@ return {
     nextnonblank={args=1, base=1},
     nr2char={args={1, 2}, base=1},
     ['or']={args=2, base=1},
-    pathshorten={args=1, base=1},
+    pathshorten={args={1, 2}, base=1},
     pow={args=2, base=1},
     prevnonblank={args=1, base=1},
     printf={args=varargs(1), base=2},