Fix: empty kotlinx.interval.datetime project

Since this project was empty, publication to Maven was failing resulting in the badge status showing as failed, even though publication of the main library was successful. I want to keep the project as a placeholder for other potential datetime interval types, e.g., `LocalDate`. Until then, I added a potentially useful type alias placeholder for backwards compatibility.

Author: Whathecode

kotlinx.interval.datetime/src/commonMain/kotlin/BackwardsCompatibility.kt

@@ -0,0 +1,6 @@
+package io.github.whathecode.kotlinx.interval.datetime
+
+
+// In version 2.2.0, `InstantInterval` was moved from this library to the main library.
+// This only provides compile-time backwards compatibility for older versions. Not binary compatibility.
+typealias InstantInterval = io.github.whathecode.kotlinx.interval.InstantInterval

kotlinx.interval/src/commonMain/kotlin/InstantInterval.kt

@@ -1,7 +1,5 @@
 package io.github.whathecode.kotlinx.interval
 
-import io.github.whathecode.kotlinx.interval.Interval
-import io.github.whathecode.kotlinx.interval.IntervalTypeOperations
 import kotlin.math.absoluteValue
 import kotlin.time.Duration
 import kotlin.time.Instant