typescript: Map.get support added

[abobr]

Fixing the issue: #74

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[mohsen1]

Can this be an extension of the Map interface?

[mohsen1]

if we add support for Map we should add support for Set too. Not sure about WeakMap and WeakSet

[yungsters]

Thanks for taking the time to contribute this pull request and sign the CLA.

However, this was intentionally unsupported by idx. IIRC, we intentionally kept the contract of idx to simple member expressions because of non-trivial considerations for order of execution, etc.

Given that optional chaining is a formal active proposal now, I'm also reluctant to increase the scope of idx. I hope this makes sense.