wasm: prevent bundlers from resolving import("node:timers")

This fixes the following esbuild error:

    ✘ [ERROR] Could not resolve "node:timers"

        www/ghc_wasm_jsffi.js:66:25:
          66 │     return (await import("node:timers")).setImmediate;
             ╵                          ~~~~~~~~~~~~~

      The package "node:timers" wasn't found on the file system but is built into node. Are you trying
      to bundle for node? You can use "--platform=node" to do that, which will remove this error.

Previously (i.e. after !13503), one had to work around this by passing
`--external:node:timers`.