feat: Add deref function

[PeterStaev]

Makes compatible with NS Core modules 8.4

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• [x] The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages.
• [x] There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
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• [ ] Tests for the changes are included.

What is the current behavior?

If you use JSC runtime with NS Core 8.4 you get errors that deref is not a function.

What is the new behavior?

Adds a deref function to WeakRef which is the same as get per the ES specs.

Fixes/Implements/Closes #[Issue Number].