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/null.markdown

@@ -1,6 +1,8 @@
 ^title Null Class
 ^category core
 
+## Methods
+
 ### **!** operator
 
 Returns `true`, since `null` is considered [false](../control-flow.html#truth).