Add acorn dependency to fix npm warning
Add acorn dependency to fix npm warning

It seems there is a problem with npm installing 'peer dependencies'
npm's warning says that we need to install this dependency ourselves.
The bug is tracked here:
https://github.com/eslint/eslint/issues/11018

alphagov/govuk_prototype_kit,dwpdigitaltech/hrt-prototype,alphagov/govuk_prototype_kit,dwpdigitaltech/hrt-prototype,dwpdigitaltech/hrt-prototype,alphagov/govuk_prototype_kit