feat: add String.fromCodePoint static method and refactor UTF-8 encoding into reusable utilities

Implement the `String.fromCodePoint(_)` primitive that creates a string from a Unicode code point, with validation for integer range 0–0x10ffff. Extract inline UTF-8 encoding logic from `readUnicodeEscape` in the compiler into new `wrenUtf8NumBytes` and `wrenUtf8Encode` utility functions in `wren_utils`, and add `wrenStringFromCodePoint` helper in `wren_value`. Update documentation for core classes to add "Methods" and "Static Methods" section headers, and include `String.fromCodePoint` docs with example. Add a test file `from_code_point.wren` covering various code points.

doc/site/core/fn.markdown

@@ -18,6 +18,8 @@ argument](../functions.html#block-arguments) to some other method.
 
 It is a runtime error if `function` is not a function.
 
+## Methods
+
 ### **arity**
 
 The number of arguments the function requires.