Add containsKey assertion for Map

[Goooler]

No description provided.

[jzbrooks]

The key assertion does this. What's the value add?

[Goooler]

Inverted for

assertk/assertk/src/commonMain/kotlin/assertk/assertions/map.kt

Lines 118 to 124 in a69843f

	/**
	* Asserts the map does not contain the expected key.
	*/
	fun <K, V> Assert<Map<K, V>>.doesNotContainKey(key: K) = given {
	if (!it.containsKey(key)) return
	expected("to not contain key:${show(key)} but had value: ${show(it[key])}")
	}

[jzbrooks]

The key assertion asserts that the map contains a key and transforms the assert into an assertion on the value. You don't have to use the latter part of that functionality if you don't want to. The former does the same thing the proposed assertion does.