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
@@ -117,6 +118,32 @@ return {
       },
     })
 
+    vim.lsp.config('harper_ls', {
+      filetypes = {}, -- Disable for all filetypes
+    })
+
+    -- (Dis|en)able harper-ls when spell mode is (dis|en)enabled.
+    vim.api.nvim_create_autocmd("OptionSet", {
+      pattern = "spell",
+      callback = function()
+        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')
     cmp.setup({
       snippet = {