feat(plugin-vite): automatically switch to esm mode when enabled

[maximsmol]

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Summarize your changes:

Electron now supports ESM by settings "type": "module" in package.json but @electron-forge/plugin-vite will output CommonJS code regardless, which will then fail to load (typically by trying to reference require).

This patch reads type from package.json and automatically changes the main vite config output format to esm if modules are requested. Note that we parse package.json twice now (the first time to look for a forge config) but this seemed cleaner than trying to pass the parsed JSON around.

I am deliberately leaving preload scripts as-is since they ignore "type": "module" and load fine with the current settings. That being said, Electron supports ESM preload scripts, so we might want to automatically change the output format there as well. This would also require renaming the output from .js to .mjs There is also the sandboxing limitation, which may mean that bundling to CommonJS is the best way for preload scripts regardless, but I do not understand well enough how people use preload scripts (and in particular how common sandboxing is) to make that decision.

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Please let me know if I need to add docs or automated tests here—the change is pretty trivial so I did not bother.

Related: #3684

[BlackHole1]

PTAL @caoxiemeihao