fix(vim.mpack): rename pack/unpack => encode/decode #16175

Problem:
1. "unpack" has an unrelated meaning in Lua:
   https://www.lua.org/manual/5.1/manual.html#pdf-unpack
2. We already have msgpackparse()/msgpackdump() and
   json_encode()/json_decode(), so introducing another name for the same
   thing is entropy.

Solution:
- Rename vim.mpack.pack/unpack => vim.mpack.encode/decode

Caveat:
This is incongruent with the `Unpacker` and `Packer` functions.
- It's probably too invasive to rename those.
- They also aren't part of our documented interface.
- This commit is "reversible" in the sense that we can always revert
  it and add `vim.mpack.encode/decode` as _aliases_ to
  `vim.mpack.pack/unpack`, at any time in the future, if we want
  stricter fidelity with upstream libmpack. Meanwhile,
  `vim.mpack.encode/decode` is currently the total _documented_
  interface of `vim.mpack`, so this change serves the purpose of
  consistent naming in the Nvim stdlib.

test/functional/lua/mpack_spec.lua

@@ -7,16 +7,16 @@ local exec_lua = helpers.exec_lua
 
 describe('lua vim.mpack', function()
   before_each(clear)
-  it('can pack vim.NIL', function()
+  it('encodes vim.NIL', function()
     eq({true, true, true, true}, exec_lua [[
-      local var = vim.mpack.unpack(vim.mpack.pack({33, vim.NIL, 77}))
+      local var = vim.mpack.decode(vim.mpack.encode({33, vim.NIL, 77}))
       return {var[1]==33, var[2]==vim.NIL, var[3]==77, var[4]==nil}
     ]])
   end)
 
-  it('can pack vim.empty_dict()', function()
+  it('encodes vim.empty_dict()', function()
     eq({{{}, "foo", {}}, true, false}, exec_lua [[
-      local var = vim.mpack.unpack(vim.mpack.pack({{}, "foo", vim.empty_dict()}))
+      local var = vim.mpack.decode(vim.mpack.encode({{}, "foo", vim.empty_dict()}))
       return {var, vim.tbl_islist(var[1]), vim.tbl_islist(var[3])}
     ]])
   end)