Re-add Neovim configuration files

Re-included from https://github.com/opdavies/opdavies.nvim.

config/neovim/plugin/lint.lua

@@ -0,0 +1,19 @@
+local lint = require "lint"
+
+lint.linters_by_ft = {
+  dockerfile = { "hadolint" },
+  javascript = { "eslint_d" },
+  json = { "jsonlint" },
+  lua = { "luacheck" },
+  markdown = { "markdownlint" },
+  nix = { "nix" },
+  php = { "php", "phpcs", "phpstan" },
+}
+local lint_augroup = vim.api.nvim_create_augroup("lint", { clear = true })
+
+vim.api.nvim_create_autocmd({ "BufEnter", "BufWritePost", "InsertLeave" }, {
+  group = lint_augroup,
+  callback = function()
+    lint.try_lint()
+  end,
+})