Wasm build configuration

Set MOON_WASM_NEW_ALLOCATOR=1 to enable the MoonBit TLSF allocator:

MOON_WASM_NEW_ALLOCATOR=1 moon build --target wasm

This applies to the wasm backend in moon build, moon run, moon test, and moon bench, including --output-wat. It passes -allocator tlsf-mbt to moonc link-core and requires a compiler that supports that option.

When unset or set to any value other than 1, the compiler’s default allocator is used. Other target backends are unaffected.