added null-ls functionality for linting, formatting and code actions (via mason)

nvim/.config/nvim/after/plugin/null-ls.lua

@@ -0,0 +1,12 @@
+local null_ls = require('null-ls')
+
+local sources = {
+    null_ls.builtins.diagnostics.trail_space.with{
+        disabled_filetypes = { "lua" }
+    },
+    null_ls.builtins.diagnostics.shellcheck,
+}
+
+null_ls.setup({
+    sources =  sources,
+})