Replace all uses of the `new` reserved word with explicit `this new()` constructor definitions and `ClassName.new()` instantiation calls in builtin/core.wren. Update all documentation files (classes.markdown, fiber.markdown, fn.markdown, sequence.markdown, error-handling.markdown, expressions.markdown, fibers.markdown, functions.markdown) to reflect the new constructor syntax, removing `new` keyword examples and replacing them with `ClassName.new()` patterns and `this new()` definitions.
Add a MAX_FIELDS constant (255) to wren_common.h and implement field overflow
validation in the compiler and VM. The compiler now emits an error when a class
declares more than 255 fields, and the VM checks for inherited field overflow
at class creation time, producing a descriptive runtime error. Also add four
test files (many_fields, many_inherited_fields, too_many_fields,
too_many_inherited_fields) to verify both valid and invalid field counts.
