Update svglint ignore list, add ignore list updater (#3433)

This updates the ignore list as per #3250 and embeds code for updating
the ignore file when the environment variable `SI_UPDATE_IGNORE=true`
is set and the linter is run like so `SI_UPDATE_IGNORE=true npm run svglint`
-- running this command will display all remaining linting failures and
use these to update the ignore file.

This also:

* Normalises the top-level object key in the ignore file to match each
  linter name
* Sorts the ignore output on top-level key and icon name value to
  make output consistent
* Speeds up icon center checking if ignored
* Ignores icons in size linter if icon is otherwise ignored