Add option `elision` to prevent warning about empty array elements

A quick attempt at making it possible to ignore this error, while still enabling it by default.

WDYT? I feel like using this feature is typically an error, unless people specifically ask for it, such as for
engine unit tests or polyfill unit tests.

BREAKING CHANGE:

In projects which do not enable ES3 mode, it is now an error by default to use elision array elements,
also known as empty array elements (such as `[1, , 3, , 5]`)

Fixes gh-2062