Remove experimental flag for Symbol.dispose

[ketanhwr]

Motivation

With the explicit resource management proposal moving to stage 4 (https://bsky.app/profile/robpalmer.bsky.social/post/3lqaieimpq222), and every major runtime and browser supporting it by default now (https://v8.dev/features/explicit-resource-management), we should remove the experimental flag WASM_BINDGEN_EXPERIMENTAL_SYMBOL_DISPOSE and generate Symbol.dispose definition by default.