Add a clone_inner method to allow cloning of inner data

[hansl]

The issue I'm looking to solve is that I'm passing back the same object into JavaScript (think callbacks). The object is reentrant safe, but I'm ending up doing a lot of this.borrow().clone(). This would make it a bit easier to manage.

The JS code looks like that:

const someValue = new ApiClass();
someValue.doSomethingAndCallback(() => {
  // Using a naive but straightforward borrow_mut(), Boa would fail here as
  // the `doSomethingElse` method would need to borrow_mut() again.
  someValue.doSomethingElse();
});

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 51.86%. Comparing base (6ddc2b4) to head (43193de).
Report is 244 commits behind head on main.

Files	Patch %	Lines
core/interop/src/lib.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3968      +/-   ##
==========================================
+ Coverage   47.24%   51.86%   +4.61%     
==========================================
  Files         476      468       -8     
  Lines       46892    45295    -1597     
==========================================
+ Hits        22154    23490    +1336     
+ Misses      24738    21805    -2933     

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[jedel1043]

Looks great! I Just have a small doc nitpick.