rts: rename prelude.js to prelude.mjs

This commit renames prelude.js to prelude.mjs for wasm backend rts
jsbits, and slightly adjusts the jsbits contents. This is for
preparing the implementation of dyld.mjs that contains wasm dynamic
linker logic, which needs to import prelude.mjs as a proper ESM
module.

(cherry picked from commit 71a471e7)