Own - have.own.property

[DaniBr]

This code is valid:
expect(element.attributes).to.have.own.property('data-a');
However, I get this type error:
test/t1.ts(42,44): error TS2339: Property 'own' does not exist on type 'Assertion'.

I'll suggest to add "own" to interface LanguageChains.

[unional]

PR welcome.
Also, chai is trying to add the typings to the library directly: chaijs/chai#822
When that happens, this and the DT version will be deprecated.