perf(ext/node): replace node: imports with core.loadExtScript for buffer and nextTick (#33862)
Replace ESM `import` of `node:buffer` and `node:process` (nextTick) in
polyfill files with direct `core.loadExtScript()` calls to the
underlying internal modules. Also simplify `internal/assert.mjs` to
return the assert function directly instead of wrapping in `{ default:
assert }`.