Make AnyFrame.renderToString public

#1129 - renderToString is currently internal, making it difficult to modify the properties of the function

Author: bwjohnson92

core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/io/string.kt

@@ -19,7 +19,7 @@ import org.jetbrains.kotlinx.dataframe.nrow
 import org.jetbrains.kotlinx.dataframe.size
 import java.math.BigDecimal
 
-internal fun AnyFrame.renderToString(
+fun AnyFrame.renderToString(
     rowsLimit: Int = 20,
     valueLimit: Int = 40,
     borders: Boolean = false,