improve Jsligo proper return handling

Motivation and Context

Improve the return handling in JsLIGO Related Issue(s): https://gitlab.com/ligolang/ligo/-/issues/1525

Description

Currently the handling of return in JsLIGO is flawed. This tries to fix some of the issues. This MR does not fix issues related to switch and while.

let a = b => { b }

will now return a unit instead of b. To return b, the return keyword should be used:

let a = b => { return b }`

The handling of blocks has also improved:

const g = n => {
  let output = n;

  {
    output += 1 ;
    if (n > 1) {
      return (output + 12)
    } 
  }

  return output
}

will now correctly work.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Performance improvement (non-breaking change that improves performance)
• None (change with no changelog)

Changelog

Improve the return handling in JsLIGO

Checklist:

• Changes follow the existing coding style (use dune @fmt to check).
• Tests for the changes have been added (for bug fixes / feature).
• Documentation has been updated.
• Changelog description has been added (if appropriate).
• Start titles under ## Changelog section with #### (if appropriate).
• There is no image or uploaded file in changelog
• Examples in changed behaviour have been added to the changelog (for breaking change / feature).