add toUser cast and fix of sourceUser

tgbotapi.core/src/commonMain/kotlin/dev/inmo/tgbotapi/utils/PrivateChatToUser.kt

@@ -0,0 +1,21 @@
+package dev.inmo.tgbotapi.utils
+
+import dev.inmo.tgbotapi.types.chat.*
+
+/**
+ * Trying to convert current [PrivateChat] to [User]
+ *
+ * * If [this] actually is [User] or some [Bot], will return this as is
+ * * If [this] is some [PreviewPrivateChat], will create new [CommonUser]
+ *
+ * !!!WARNING!!! The returned [User] CAN BE NOT EQUAL to user from some
+ * [dev.inmo.tgbotapi.types.message.abstracts.ContentMessage] due to absence of some fields (like premium flag or
+ * language)
+ */
+fun PrivateChat.toUser(): User = when (this) {
+    is ExtendedPrivateChatImpl -> CommonUser(id, firstName, lastName, username)
+    is CommonUser -> this
+    is CommonBot -> this
+    is PrivateChatImpl -> CommonUser(id, firstName, lastName, username)
+    is ExtendedBot -> this
+}