Allow Stringable variables in if statements

[passchn]

This PR enables users to evaluate variables that are of type \Stringable in an if statement:

{{#if myVariable}}
  Shown when (string) $myVariable is a true-ish string.
{{/if}}

[theodorejb]

Thank you @passchn!

[passchn]

Thank you for merging the PR so quickly @theodorejb :) Do you plan to tag a new version quite soon?

[theodorejb]

@passchn I just tagged v0.9.9!