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.
Add a public `arity` getter to the Fn class that returns the number of required arguments for a function. Internally rename the `numParams` field to `arity` across the codebase for consistency, update the C API parameter name from `numParams` to `arity` in `wrenDefineMethod` and `wrenDefineStaticMethod`, and adjust the error message in `callFunction` to use the new field name. Also update documentation to clarify that extra arguments are ignored rather than causing an error, and add a new test file `test/function/arity.wren` verifying arity values from 0 to 4.
Bob Nystrom