Update FatalErrorPage codemod to work with hybrid JS/TS projects

[Tobbe]

This PR makes the fatalerrorpage codemod look for an existing FatalErrorPage.{js,tsx} file and replaces that, instead of looking for a tsconfig.json file.

Background

I just upgraded one of my RW projects from v0.45 to v0.48

I verified that it still worked after running yarn rw upgrade

Then I applied the FatalErrorPage codemod and got this

And in my terminal I see

web | ERROR in ./src/Routes.js 35:6
web | Module parse failed: Identifier 'FatalErrorPage' has already been declared (35:6)
web | File was processed with these loaders:
web |  * ../node_modules/@pmmmwh/react-refresh-webpack-plugin/loader/index.js
web |  * ../node_modules/babel-loader/lib/index.js
web | You may need an additional loader to handle the result of these loaders.
web | |   loader: () => import("./pages/FatalErrorPage/FatalErrorPage")
web | | };
web | > const FatalErrorPage = {
web | |   name: "FatalErrorPage",
web | |   loader: () => import("./pages/FatalErrorPage/FatalErrorPage")
web |  @ ./src/App.js 12:0-30 30:269-275
web |  @ ./src/index.js 9:0-24 29:53-56 31:52-55
web |
web | webpack 5.70.0 compiled with 1 error in 2696 ms

I noticed I had a new FatalErrorPage.tsx file. So now I had two error pages.

Deleting the old FatalErrorPage.js file makes everything work again.

This project of mine started its life as a JS RW project, but I've gradually been moving to TS, that's why I had a .js error page and a tsconfig.json file.

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