refactor(create-electron): Changed names for electron main and preload files

[NaspoDev]

Description

I find that the project structure of electron apps would be a lot more clear if the main process file is named main.js, and the preload file named preload.js. Currently, they are both named index.js, which is a bit confusing/unorganized.

This pull request has, for all frameworks:

• Changed the names of all main process files from index.js/ts to main.js/ts.
• Changed the names of all preload files from index.js/ts to preload.js/ts. (This includes preload.d.ts).
• Updated the BrowserWindow's webPreferences.preload option to reflect the name change in the path.
• Updated the package.json main file name.

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the Commit Convention.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).