feat(plugin-vite): enable main process hot restart via exported API

[bglgwyng]

Add restartApp()/onAppRestart() to @electron-forge/core-utils as an explicit API for triggering Electron app restarts. The Vite plugin calls restartApp() in its closeBundle hook when the main process bundle is rebuilt. The start API registers the actual restart logic via onAppRestart(). The existing rs stdin handler also goes through restartApp() now, so there's a single restart path.

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Summarize your changes:

Background

Main process hot restart for the Vite plugin has been requested and attempted multiple times:

• feat(plugin-vite): hot-restart 🔥 #3203 — @caoxiemeihao's initial hot-restart PR. It used process.stdin.emit('data', 'rs') to trigger restarts from the plugin, which reviewers flagged as a hack that should be replaced by an API exported from @electron-forge/core.
• feat(vite-plugin): auto restart after rebuild Main process #3326 — A follow-up scoped to hot restart only. Blocked on the same process.stdin coupling issue.
• refactor(core): allow restart logic to be triggered using an event #3380 — @erikian proposed a dedicated EventEmitter-based approach to decouple restart signaling from process.stdin, but the PR was closed before merging.
• feat(plugin-vite): better logic #3583 (merged) — Brought pluginHotRestart('restart') into @electron-forge/plugin-vite, but the restart branch was left commented out with // TODO: blocked in #3380 because no cross-package restart API existed yet.

What this PR does

Implements the approach suggested in #3380 and unblocks the commented-out code in #3583:

1. Adds restartApp() and onAppRestart() to @electron-forge/core-utils, backed by a module-scoped EventEmitter. Plugins call restartApp() to signal a restart; the start API registers the actual kill→respawn logic via onAppRestart().
2. Replaces the commented-out process.stdin.emit('data', 'rs') in pluginHotRestart with restartApp().
3. The existing rs stdin handler also calls restartApp() instead of duplicating the restart logic, so both paths converge.

[bglgwyng]

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