nvim/diagnostic: clobber repeated diagnostic floats

Due to the way we render `vim.diagnostic.open_float/1` on every `CursorHold`
event, this window will open over other floats that a user (may have)
opened, particularly `keys.Buffer.Lsp.N.ShowDocumentation`, which can be
frustrating.

This commit changes the behaviour to open the diagnostic float once per
cursor location, as this is probably what the user wants for the most
part. It is still possible for a user to run into the scenario where
they move to a diagnostic and open a float only for it to be replaced
on the `CursorHold`, but this should be much more infrequent.

Credit to github.com/wookayin for explaining how to do this.
See:
  wookayin/dotfiles@ee687b036c
  /nvim/lua/config/lsp.lua
  #L249-L263

nvim/lua/psoxizsh/diagnostic.lua

@@ -11,6 +11,19 @@ local sign = function(o)
   })
 end
 
+local DiagnosticFloat = function()
+  -- current, last diagnostic cursor position
+  local current = vim.api.nvim_win_get_cursor(0)
+  local last = vim.w.diagnostics_last_cursor or { nil, nil }
+
+  -- Show the popup diagnostics window,
+  -- but only once for the current cursor location (unless moved afterwards).
+  if not (current[1] == last[1] and current[2] == last[2]) then
+    vim.w.diagnostics_last_cursor = current
+    D.open_float({ focusable = false, scope = 'cursor' })
+  end
+end
+
 function M.config()
   -- Setup signs
   sign({ name = 'DiagnosticSignError', text = '' })
@@ -44,7 +57,7 @@ end
 
 function M.autocmds()
   au.PsoxDiagnosticHover {
-    { 'CursorHold', '*', function() D.open_float({ focusable = false, scope = 'cursor' }) end }
+    { 'CursorHold', '*', DiagnosticFloat }
   }
 end