feat: add script-file input for lint-friendly external JS scripts

Closes #714

Inline `script` is a YAML string — invisible to linters and IDEs.
The common workaround was wrapping a `require` call inside the inline script,
which still needs boilerplate and assumes a path convention.

New optional `script-file` input that accepts a path to a JS file.
The file must `module.exports` an async function receiving the standard
IoC dependency bag (`github`, `octokit`, `getOctokit`, `context`, `core`,
`exec`, `glob`, `io`, `require`).

```yaml
- uses: actions/checkout@v4
- uses: actions/github-script@v9
  with:
    script-file: .github/scripts/my-script.js
```

`script` and `script-file` are mutually exclusive — exactly one must be provided.
Relative paths resolve against `$GITHUB_WORKSPACE`; absolute paths are used as-is.

- `action.yml` — adds `script-file` input; makes `script` optional
- `src/script-file.ts` — path resolution and script loading logic
- `src/args.ts` — `AsyncFunctionArguments` extracted from `async-function.ts` so neither execution path depends on the other
- `src/main.ts` — mutual-exclusion validation; dispatches to the right execution path
- `types/non-webpack-require.ts` — corrects `__non_webpack_require__` type from deprecated `NodeRequire` / wrong `NodeJS.RequireResolve` to `NodeJS.Require`
- `__test__/script-file.test.ts` — 10 tests covering path resolution, arg forwarding, error cases
- `README.md` — new `## Script file` section with usage, IoC bag table, path resolution rules
- `.github/fixtures/script-file/` — fixture JS files for integration tests
- `.github/workflows/integration.yml` — 10 new integration test jobs: happy path (relative path, absolute path, all IoC args, json/string encoding,
  require-in-file) and error cases (both inputs set, neither set, nonexistent file, non-function export, file:// protocol)

.github/fixtures/script-file/all-args.js

@@ -0,0 +1,5 @@
+module.exports = async ({github, octokit, getOctokit, context, core, exec, glob, io, require}) => {
+  return [github, octokit, getOctokit, context, core, exec, glob, io, require]
+    .map(arg => typeof arg)
+    .every(t => t === 'function' || t === 'object')
+}