Update svglint ignore list, add ignore updater

This updates the ignore list as per simple-icons#3250 and embeds code for updating
the ignore file when the environment variable `SI_UPDATE_IGNORE=true`
is set.

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