nvim/plug: add null-ls.nvim@main, cspell.nvim@main

These gives us back the ability to use cspell, which was lost in the
transition from coc-nvim to our current lua based setup.

However, it requires the following to take effect:

1. Have nodejs installed (nvm is fine)
2. Run ':MasonInstall cspell'
3. Run ':PackerCompile'
4. Restart

All goes well, when you next open nvim, you should have cspell powered
spell checking and :LspInfo should show null-ls running.

Word substitutes triggered via:
  @Buffer.Lsp.N.Leader.CodeAction

Next/Prev word (or other diagnostic) via:
  @Global.N.DiagnosticNext
  @Global.N.DiagnosticPrev

Where @ = nvim/lua/psoxizsh/key/bind.lua

nvim/lua/psoxizsh/plugins/config/null-ls.lua

@@ -0,0 +1,58 @@
+return function()
+  local nls, util = require 'null-ls', require 'psoxizsh.util'
+
+  local S = {
+    builtin = nls.builtins,
+    cspell = require 'cspell',
+  }
+
+  local flatten = function(list)
+    local out = {}
+
+    for _, l in ipairs(list) do
+      for _, v in ipairs(l) do
+        table.insert(out, v)
+      end
+    end
+
+    return out
+  end
+
+  local if_exec = function(args)
+    return vim.fn.executable(args[1]) == 1 and vim.list_slice(args, 2) or {}
+  end
+
+  local cspell_event_handler = function(cfg, params, action)
+    if action == 'add_to_json' and vim.fn.executable('jq') == 1 then
+      local cmd = "cat %s | jq -S '.words |= sort' | tee %s > /dev/null"
+      os.execute(cmd:format(cfg, cfg))
+    end
+  end
+
+  local cspell_post_handler = function(diagnostic)
+    local level, levels = diagnostic.severity, vim.diagnostic.severity
+
+    -- Cap maximum severity of spelling mistakes
+    if level == levels.ERROR or level == levels.WARN then
+      diagnostic.severity = levels.INFO
+    end
+  end
+
+  local cspell_options = {
+    config = {
+      on_success = cspell_event_handler,
+    },
+    diagnostics_postprocess = cspell_post_handler
+  }
+
+  local defaults = {
+    sources = flatten {
+      if_exec { 'cspell',
+        S.cspell.diagnostics.with(cspell_options),
+        S.cspell.code_actions.with(cspell_options),
+      },
+    },
+  }
+
+  nls.setup(util.mconfig('config.null-ls', defaults))
+end

nvim/lua/psoxizsh/plugins/plug.lua

@@ -120,6 +120,19 @@ local plugins = {
   { 'luochen1990/rainbow' },
   { 'sheerun/vim-polyglot' },
 
+  -- Framework for integrating non-LSP sources into nvim's LSP framework
+  { 'jose-elias-alvarez/null-ls.nvim',
+      as = 'null-ls',
+      branch = 'main',
+      requires = { 'nvim-lua/plenary.nvim' },
+      after = 'cspell',
+      config = require 'psoxizsh.plugins.config.null-ls',
+  },
+  { 'davidmh/cspell.nvim',
+      as = "cspell",
+      branch = 'main',
+  },
+
   -- Maker / diagnostics listing
   { 'folke/trouble.nvim',
       requires = { 'kyazdani42/nvim-web-devicons' },