Add a CI check for todo! and FIXME comments (#21950)

Motivation for this is to support writing comments that will certainly
be revisited before merge.

Release Notes:

- N/A

script/check-todos

@@ -0,0 +1,11 @@
+#!/usr/bin/env bash
+
+set -euo pipefail
+
+# Brackets are used around characters so these don't show up in normal search.
+pattern='tod[o]!|FIXM[E]'
+result=$(git grep --no-color --ignore-case --line-number --extended-regexp -e $pattern || true)
+echo "${result}"
+if [[ -n "${result}" ]]; then
+  exit 1
+fi