tools: add script to update ESLint

Provide a bash script for updating ESLint in the project.

Backport-PR-URL: #14830
PR-URL: #13895
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>

tools/update-eslint.sh

@@ -0,0 +1,31 @@
+#!/usr/bin/env bash
+
+# Shell script to update ESLint in the source tree to the latest release.
+
+# Depends on npm and node being in $PATH.
+
+# This script must be be in the tools directory when it runs because it uses
+# $BASH_SOURCE[0] to determine directories to work in.
+
+cd "$( dirname "${BASH_SOURCE[0]}" )"
+rm -rf eslint
+mkdir eslint-tmp
+cd eslint-tmp
+
+npm install --global-style --no-binlinks --production eslint@latest
+cd node_modules/eslint
+
+# eslint-plugin-markdown is pinned at 1.0.0-beta.4 until there is a release
+# that fixes https://github.com/eslint/eslint-plugin-markdown/issues/69.
+npm install --no-bin-links --production eslint-plugin-markdown@1.0.0-beta.4
+cd ../..
+
+# Install dmn if it is not in path.
+type -P dmn || npm install -g dmn
+
+# Use dmn to remove some unneeded files.
+dmn -f clean
+
+cd ..
+mv eslint-tmp/node_modules/eslint eslint
+rm -rf eslint-tmp/