Delay TimeZone from static initialization

With defaultTimeZone initialized statically, any call to ConvertKt, even not related to TZ, triggers loading kotlinx.datetime dependency. We use ConvertKt in plugin, but including this dependency is not desirable

Author: koperagen

core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/impl/api/convert.kt

@@ -725,7 +725,7 @@ internal fun Instant.toLocalDate(zone: TimeZone = defaultTimeZone) = toLocalDate
 
 internal fun Instant.toLocalTime(zone: TimeZone = defaultTimeZone) = toLocalDateTime(zone).time
 
-internal val defaultTimeZone = TimeZone.currentSystemDefault()
+internal val defaultTimeZone get() = TimeZone.currentSystemDefault()
 
 internal fun Number.toBigDecimal(): BigDecimal =
     when (this) {