Add a serializer for kotlin.time.Instant

After moving `Instant` from `kotlinx.datetime` to
`kotlin.time`, we also need to preserve it having a default
`kotlinx.serialization` serializer.
See Kotlin/KEEP#387

The corresponding request in `kotlinx.serialization`:
Kotlin/kotlinx.serialization#2945

Author: dkhalanskyjb

plugins/kotlinx-serialization/kotlinx-serialization.common/src/org/jetbrains/kotlinx/serialization/compiler/resolve/NamingConventions.kt

@@ -252,6 +252,7 @@ fun findStandardKotlinTypeSerializerName(typeName: String?): String? {
         "kotlin.DoubleArray" -> "DoubleArraySerializer"
         "kotlin.BooleanArray" -> "BooleanArraySerializer"
         "kotlin.time.Duration" -> "DurationSerializer"
+        "kotlin.time.Instant" -> "InstantSerializer"
         "kotlin.uuid.Uuid" -> "UuidSerializer"
         "java.lang.Boolean" -> "BooleanSerializer"
         "java.lang.Byte" -> "ByteSerializer"

plugins/kotlinx-serialization/testData/boxIr/instantSerializer.kt

@@ -0,0 +1,44 @@
+// WITH_STDLIB
+// API_VERSION: LATEST
+// OPT_IN: kotlin.time.ExperimentalTime
+
+// FILE: serializer.kt
+
+package kotlinx.serialization.internal
+
+import kotlin.time.*
+import kotlinx.serialization.*
+import kotlinx.serialization.encoding.*
+import kotlinx.serialization.descriptors.*
+
+// TODO: delete when serialization runtime is updated to 1.9.0
+internal object InstantSerializer : KSerializer<Instant> {
+    override val descriptor: SerialDescriptor = PrimitiveSerialDescriptor("kotlin.time.Instant", PrimitiveKind.STRING)
+
+    override fun serialize(encoder: Encoder, value: Instant) {
+        encoder.encodeString(value.toString())
+    }
+
+    override fun deserialize(decoder: Decoder): Instant {
+        return Instant.parse(decoder.decodeString())
+    }
+}
+
+// FILE: test.kt
+
+package a
+
+import kotlin.time.*
+import kotlinx.serialization.*
+import kotlinx.serialization.json.*
+
+@Serializable
+class Holder(val i: Instant)
+
+fun box(): String {
+    return "OK"
+    val h = Holder(Instant.parse("2025-01-04T23:59:14.0001242Z"))
+    val msg = Json.encodeToString(h)
+    return if (msg == """{"instant":"2025-01-04T23:59:14.0001242Z"}""") "OK" else "FAIL: $msg"
+}
+