Add autofixer to simple-unless rule for simplest case

[godric3]

Part of #2571.

For start auto fix for simplest case of:

{{#unless condition}}
  A
{{else}}
  B
{{/unless}}

fixed to

{{#if condition}}
  B
{{else}}
  A
{{/if}}

For other cases I will need to dive more into ast manipulations

[bmish]

Awesome, thanks!