drop keepOutputType support

[harttle]

Historically we supported this to render only one tag and in that case the output should be exactly that tag returned. This results in return type of render functions need to be any or unknown (#832).

It turns out to be not a LiquidJS main use case, and we can still supporting it w/o type:

• Always set type of render functions string
• Coerce render result to string only when combining, i.e. there's multiple templates in one pass
  • This allows the use case still working w/o type safety