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/fiber.markdown

@@ -13,6 +13,8 @@ fiber is run. Does not immediately start running the fiber.
       IO.print("I won't get printed")
     }
 
+## Static Methods
+
 ### Fiber.**current**
 
 The currently executing fiber.
@@ -69,6 +71,8 @@ Similar to `Fiber.yield` but provides a value to return to the parent fiber's
 
     IO.print(fiber.call()) // "value"
 
+## Methods
+
 ### **call**()
 
 **TODO**