Update all fiber test files to replace `Fiber.create(fn { ... })` with `Fiber.create { ... }` block syntax, and add compiler support for parsing block bodies without requiring a newline after the opening brace. The compiler changes include removing `TOKEN_LEFT_BRACE` from the newline-discarding token list and adding `match(compiler, TOKEN_LINE)` calls in `finishBlock`, `methodCall`, `function`, and `block` to handle empty block bodies and inline expressions.
Add a new `isDone` native method to the Fiber class that returns true when the fiber has no remaining frames. Replace TODO comments with proper runtime error handling in `fiber_run`, `fiber_run1`, `fiber_yield`, and `fiber_yield1` to prevent running finished fibers or yielding from the main fiber. Include test cases verifying the new property and error conditions.
