Add an API function to calculate the UTF-8 encoded string size from Jerry string. (#1450)

JerryScript-DCO-1.0-Signed-off-by: Robert Sipka rsipka.uszeged@partner.samsung.com

docs/02.API-REFERENCE.md

@@ -1150,6 +1150,43 @@ jerry_get_string_size (const jerry_value_t value);
 - [jerry_get_string_length](#jerry_get_string_length)
 
 
+## jerry_get_utf8_string_size
+
+**Summary**
+
+Get the size of an utf8-encoded string. Returns zero, if the value parameter is not a string.
+
+*Note*: The difference from [jerry_get_string_size](#jerry_get_string_size) is that it returns with utf-8 string size
+instead of the cesu-8 string size.
+
+**Prototype**
+
+```c
+jerry_size_t
+jerry_get_utf8_string_size (const jerry_value_t value);
+```
+- `value` - api value
+- return value - number of bytes in the buffer needed to represent the utf8-encoded string.
+
+**Example**
+
+```c
+{
+  const jerry_char_t char_array[] = "a string";
+  jerry_value_t string = jerry_create_string (char_array);
+
+  jerry_size_t string_size = jerry_get_utf8_string_size (string);
+
+  ... // usage of string_size
+
+  jerry_release_value (string);
+}
+```
+
+**See also**
+
+- [jerry_create_string_from_utf8](#jerry_create_string_from_utf8)
+
 ## jerry_get_string_length
 
 **Summary**