Add harper-ls for spelling & grammer checking

This language server's default list of supported filetypes is
overridden, instead it is not automatically enabled but rather
(dis|en)abled when the `spell` setting is changed.

lua/plugins/completions.lua

@@ -3,6 +3,7 @@ local ensure_installed = {
   'clangd',    -- C/C++
   'lua_ls',    -- Lua
   'opencl_ls', -- OpenCL
+  'harper_ls', -- Spelling & grammar
 }
 
 if vim.fn.executable('npm') == 1 then
@@ -88,12 +89,17 @@ return {
         require('cmp_nvim_lsp').default_capabilities()),
     }
 
-    local lspconfig_custom_opts = {
+    require('mason').setup()
-      clangd = {
+    require('mason-lspconfig').setup({
-        cmd = { 'clangd', '--completion-style=detailed' },
+      automatic_installation = false,
-      },
+      ensure_installed = ensure_installed,
+    })
 
-      lua_ls = {
+    vim.lsp.config('clangd', {
+      cmd = { 'clangd', '--completion-style=detailed' },
+    })
+
+    vim.lsp.config('lua_ls', {
       settings = {
         Lua = {
           diagnostics = {
@@ -102,28 +108,40 @@ return {
           },
         },
       },
-      },
+    })
 
-      pyright = {
+    vim.lsp.config('pyright', {
       settings = {
         pyright = {
           disableOrganizeImports = true,
         },
       },
-      },
+    })
-    }
 
-    require('mason').setup()
+    vim.lsp.config('harper_ls', {
-    require('mason-lspconfig').setup({
+      filetypes = {}, -- Disable for all filetypes
-      automatic_installation = false,
+    })
-      ensure_installed = ensure_installed,
+
-      handlers = {
+    -- (Dis|en)able harper-ls when spell mode is (dis|en)enabled.
-        function(server_name)
+    vim.api.nvim_create_autocmd("OptionSet", {
-          local opts = vim.tbl_deep_extend("force",
+      pattern = "spell",
-            lspconfig_default_opts, lspconfig_custom_opts[server_name] or {})
+      callback = function()
-          require('lspconfig')[server_name].setup(opts)
+        local bufnr = vim.api.nvim_get_current_buf()
+        if vim.v.option_new then
+          vim.lsp.start({
+            name = "harper_ls",
+            cmd = { "harper-ls", "--stdio" },
+            root_dir = vim.fn.getcwd(),
+          }, { bufnr = bufnr })
+        else
+          for _, client in pairs(vim.lsp.get_clients({ bufnr = bufnr })) do
+            if client.name == "harper_ls" then
+              vim.lsp.buf_detach_client(bufnr, client.id)
+              break
+            end
+          end
+        end
       end,
-      },
     })
 
     local cmp = require('cmp')