add missing dependency warning for ripgrep

For live_grep and grep_string pickers.

lua/telescope/builtin/__files.lua

@@ -36,6 +36,22 @@ local escape_chars = function(string)
   })
 end
 
+local has_rg = function(picker_name)
+  if vim.fn.executable "rg" == 1 then
+    return true
+  end
+
+  utils.notify(picker_name, {
+    msg = string.format(
+      "'ripgrep' is a required dependency for the %s picker. "
+        .. "Visit https://github.com/BurntSushi/ripgrep#installation for installation instructions.",
+      picker_name
+    ),
+    level = "ERROR",
+  })
+  return false
+end
+
 local get_open_filelist = function(grep_open_files, cwd)
   if not grep_open_files then
     return nil
@@ -93,6 +109,10 @@ end
 --  opts.search_dirs -- list of directory to search in
 --  opts.grep_open_files -- boolean to restrict search to open files
 files.live_grep = function(opts)
+  if not has_rg "live_grep" then
+    return
+  end
+
   local vimgrep_arguments = opts.vimgrep_arguments or conf.vimgrep_arguments
   local search_dirs = opts.search_dirs
   local grep_open_files = opts.grep_open_files
@@ -166,6 +186,10 @@ files.live_grep = function(opts)
 end
 
 files.grep_string = function(opts)
+  if not has_rg "grep_string" then
+    return
+  end
+
   opts.cwd = opts.cwd and vim.fn.expand(opts.cwd) or vim.loop.cwd()
   local vimgrep_arguments = vim.F.if_nil(opts.vimgrep_arguments, conf.vimgrep_arguments)
   local word