Implement inverse blocks

[thekid]

A ^ starting a block is an inverse block. Example:

Long version:
{{#person}}User {{name}}{{/person}}{{^person}}Guest{{/person}}

Shorter version:
{{#person}}User {{name}}{{^}}Guest{{/person}}

This expression yields:

• "User test" for person=["name" => "test"]
• "Guest" for person=null

This fixes the MethodNotImplementedException being raised for these expressions.

See https://handlebars-lang.github.io/spec/#prod-InvertedBlockStart

[thekid]

Released in https://github.com/xp-forge/handlebars/releases/tag/v9.2.0