This adds a new script for linting WebIDL files, and adds it to the set
of scripts Meta/lint-ci.sh runs. Initially, this script does just one
thing: normalizes IDL definition lines so they start with four spaces.
(cherry picked from commit a7578164d4ea2aa6e7831d83e9a55b1ff39e37f7;
minorly amended to resolve conflict in lint-ci.sh due to serenity still
having check-emoji.py and check-markdown.sh)
