add utf8 library docs

doc/en/scripting/builtins/libutf8.md

@@ -0,0 +1,18 @@
+# *utf8* library
+
+The library provides functions for working with UTF-8.
+
+```lua
+-- Converts a UTF-8 string to a Bytearray or an array of numbers if
+-- the second argument is true
+utf8.tobytes(text: str, [optional] usetable=false) -> Bytearray|table
+
+-- Converts a Bytearray or an array of numbers to a UTF-8 string
+utf8.tostring(bytes: Bytearray|table) -> str
+
+-- Returns the length of a Unicode string
+utf8.length(text: str) -> int
+
+-- Returns the code of the first character of the string
+utf8.codepoint(chars: str) -> int
+```