Add "effect" as a parameter in handle block

[macabeus]

We need to add a parameter on handle block to assign the effect variable, as like we already have on catch block:

try {
  // ...
} handle (effect) {
  // ...
}

This is a good suggestion that some guys said on the following comments:

• Alternative Syntax #14 (comment)
• How will the effect interact with the ecosystem of TypeScript? #13 (comment)

[macabeus]

There is a nice discussion here that conflicts with this issue: #8 (comment)

[macabeus]

Done: #18