Runtime: only install the uncaughtException handler for standalone programs

[hhugo]

Fixes #1277.

Problem

The runtime registers a process-wide uncaughtException handler at module-load time (runtime/js/sys.js):

process.on("uncaughtException", function (err, origin) {
  caml_fatal_uncaught_exception(err);
  process.exit(2);
});

This is meant to emulate native OCaml's fatal-error behaviour (print Fatal error: exception …, exit with code 2). But it runs unconditionally, even when the generated code is loaded as a library. As reported in #1277, requiring jsoo output in the Node REPL then overrides the host's error handling: an error completely unrelated to the OCaml code (e.g. a typo at the prompt) trips the handler and aborts the whole process via process.exit(2) instead of letting the REPL recover.

Fix

Only install the handler when the program is run directly (require.main === module, e.g. node a.js), not when it is loaded via require("./a.js") or in the REPL. module/require are already in scope inside the runtime (the runtime uses require("node:fs") directly), so the check works through the IIFE wrapper. When the discriminator is unavailable (no CommonJS module, e.g. ESM/browser), the previous behaviour is preserved.

This matches the direction suggested on the issue: keep the fatal-error logic for standalone programs only.

The same guard is applied to the wasm_of_ocaml runtime (runtime/wasm/runtime.js).

Behaviour after the fix

• node a.js — handler installed; an async uncaught exception still prints Fatal error: exception … and exits 2 (unchanged).
• require("./a.js") (library / REPL) — handler not installed; the host's own error handling applies, so unrelated errors no longer abort the process and the REPL recovers normally.

Tests

Added a regression test in compiler/tests-compiler/error.ml that compiles a module, loads it from a driver via require, and asserts process.listenerCount("uncaughtException") === 0.