Cannot pass <:else> or <:inverse> block

[chancancode]

• Confirm if this is intended to be possible
• Confirm if they should be aliases (see Aliases #2)
• Confirm if they should be allowed to have block params: Yes
• Add tests upstream
• Implement feature in polyfill

If this is to be implemented, they probably should be rewritten as vanilla {{else}} blocks for maximum compatibility. However, this would require rewriting the component invocation using curly invocation, and therefore cannot pass attributes and modifiers. Not sure if there is a good work around for this.