adjust completion list keymaps

disable up/down arrow key navigation in the completion list. up/down
arrow keys will instead close the list. this helps reduce the feeling
of interruption i sometimes get when a completion list is open but i
(for whatever reason) need to navigate away from the current cursor
position, which would otherwise require either accepting the completion
or continuing to type at the cursor, or cancelling the completion with
the escape key.

it's a small thing, but this helps reduce some annoyances/frustrations
with auto-completion by having less overloading of the cursor keys
while in insert mode

ctrl-j and ctrl-k now behave identically to the old completion list
up/down arrow key mappings.

also the escape key mapping now performs an abort instead of just
closing the list. this feels a little bit more appropriate, but i'll
probably have to try this out for a while and see if i feel like it is
an improvement or just extra confusion.

lua/plugins/nvim-cmp.lua

@@ -39,6 +39,7 @@ return {
     config = function()
       -- See `:help cmp`
       local cmp = require 'cmp'
+      local types = require 'cmp.types'
       local luasnip = require 'luasnip'
       luasnip.config.setup {}
 
@@ -55,6 +56,15 @@ return {
         --
         -- No, but seriously. Please read `:help ins-completion`, it is really good!
         mapping = cmp.mapping.preset.insert {
+          -- Disable use of <Up> and <Down> for navigation within a completion list.
+          ['<Down>'] = cmp.mapping.close(),
+          ['<Up>'] = cmp.mapping.close(),
+
+          -- Identical to old <Down> key behavior
+          ['<C-j>'] = cmp.mapping.select_next_item { behavior = types.cmp.SelectBehavior.Select },
+          -- Identical to old <Up> key behavior
+          ['<C-k>'] = cmp.mapping.select_prev_item { behavior = types.cmp.SelectBehavior.Select },
+
           -- Select the [n]ext item
           ['<C-n>'] = cmp.mapping.select_next_item(),
           -- Select the [p]revious item
@@ -69,7 +79,8 @@ return {
           --  This will expand snippets if the LSP sent a snippet.
           ['<C-y>'] = cmp.mapping.confirm { select = true },
 
-          ['<Esc>'] = cmp.mapping.close(),
+          -- Cancel/abort any selected/partial completion and close the completion list.
+          ['<Esc>'] = cmp.mapping.abort(),
 
           -- If you prefer more traditional completion keymaps,
           -- you can uncomment the following lines