Add a programmable switch for backtrace

Currently, backtrace can only be enabled with "RUST_BACKTRACE" environment variables, while it is unsupported to set environment variables with `std::env::set_var` on wasm platforms.
There are some other ways to set environment variables on wasm, however sometimes they are impossible, for example, in cloudflare workers, they don't provide a way to set wasm environment variables.