fix(template-webpack-typescript): use fork-ts-checker-webpack-plugin in main webpack config

[NoahAndrews]

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Summarize your changes:

This PR modifies the the webpack-typescript template to enable type-checking in the main webpack config, rather than just the renderer's webpack config. Without this change, Typescript compile errors in both the renderer and main process Typescript files did not appear to get printed anywhere at all. @MarshallOfSound did you verify that type-checking worked in the original version of the typescript-webpack template?

With this change, Typescript errors in either the renderer or main process source code get printed to the console that npm start is launched from.