feat: add IndexMap::get_key_value_mut

[ya7010]

Added a method whose implementation had been forgotten.

[cuviper]

Nit: the title and commit message left out the _mut suffix.

This is a simple thing to add, but I notice that only hashbrown has this map method, not the standard library's HashMap nor BTreeMap. So it's not really missing or forgotten in terms of std parity, but we should probably add this everywhere. Would you be willing to propose this as an ACP?

In the meantime, IndexMap::get_full_mut provides the same access, along with the index.

[cuviper]

There's some prior discussion on rust-lang/rust#49347, but not much to go on.