Moving to neovim 0.11, new calc, keys streamlined

Keybindings:

- <space>E (explain error) now is <space>x
- dropped <C-W>0 and following, were remainders from Emacs
- insert: <C-Del> now deletes word right of cursor

LSP:

- diagnostics popup replaced with neovim 0.11 virtual lines (much nicer)

Options:

- neo-tree width dropped, was too wide
- workspaces no longer auto open, was interfering with using neovim as
  git client or file manager

Plugins:

- replaced cmp-rpncalc with qalc.nvim (which uses the qalc cli)

after/ftplugin/rust.lua

@@ -2,7 +2,7 @@ local bufnr = vim.api.nvim_get_current_buf()
 
 vim.keymap.set(
   "n",
-  "<space>E",
+  "<space>x",
   function()
     vim.cmd.RustLsp({ 'explainError', 'current' })
   end,