Add support for each with block params

[thekid]

Example:

<!-- Using block params -->
{{#each items as |item index|}}
  {{index}}: {{item.name}}
{{/each}}

<!-- Equivalent -->
{{#each items}}
  {{@index}}: {{name}}
{{/each}}

See handlebars-lang/handlebars.js#906

[thekid]

Released in https://github.com/xp-forge/handlebars/releases/tag/v6.0.0