ci: Add shellcheck for scripts (#20631)

Fixes shellcheck errors in script/*
Adds a couple trailing newlines.
Adds `script/shellcheck-scripts` and associated CI machinery.
Current set ultra-conservative, does not output warnings, only errors.

script/shellcheck-scripts

@@ -0,0 +1,12 @@
+#!/usr/bin/env bash
+
+set -euo pipefail
+
+mode=${1:-error}
+[[ "$mode" =~ ^(error|warning)$ ]] || { echo "Usage: $0 [error|warning]"; exit 1; }
+
+cd "$(dirname "$0")/.." || exit 1
+
+find script -maxdepth 1 -type f -print0 |
+  xargs -0 grep -l -E '^#!(/bin/|/usr/bin/env )(sh|bash|dash)' |
+  xargs -r shellcheck -x -S "$mode" -C