vm2: NodeVM custom resolution bypasses external path boundaries

## Summary At source revision `91034466bfb7f56b95fd48083ec6ca36d058f164` of vm2 3.11.8, an untrusted `NodeVM` guest can turn one allowlisted custom-resolved module into authorization for a separate file whose path merely shares the resolved path's string prefix. `LegacyResolver.customResolve` stores the resolved path in `this.externals` as `^` without an end or separator boundary; a later absolute require of a sibling such as `foo2/index.js` therefore passes the external check and is loaded through `hostRequire` when the configured context is `host`. The decisive attack loaded `foo` as `FOO_OK` and then executed the prefix-sharing sibling, which returned `PREFIX_PWN` after invoking `child_process`; an otherwise identical control denied the sibling with `ENOTFOUND`. ## Technical Details The affected configuration is a documented `NodeVM` use in which the embedder sets `require.external` to `{modules: ['foo'], transitive: false}`, supplies a custom resolver that returns the `foo` directory, sets a root directory, and uses `context: 'host'`. The guest controls the `require` specifiers and requests the allowlisted bare name before requesting the absolute path of the prefix-sharing sibling. The test entry point is deliberately outside the configured root so ordinary `node_modules` lookup misses and the custom resolver is consulted. The source-to-sink path is: - `NodeVM.run` executes guest code and creates the module-specific `require` function at `lib/nodevm.js:516-575`. - `Defaul