Update acorn for ECMAScript 2020 features

[vnc5]

This updates acorn to the latest version, specifies a default ecmaVersion which is required now and a test for the nullish coalescing operator which is the reason for this pull request.

I'm hoping for a bump in pug-lexer and pug too, so that we can use the new features in our templates.

Being able to div= 5 ?? 6 would shrink my templates immensely.

[rollingversions]

Change Log for is-expression (4.0.0 → 5.0.0)

Breaking Changes

• Upgrade acorn from 7.1.1 to 8.0.1

  This changes the default mode to ES2020, meaning many things now count as valid expressions that were previously considered errors.

Edit changelog

[cray0000]

we can just specify

ecmaVersion: 'latest'

to be always up to date