Update "preload.js" to "preload.ts" in main.ts

[azn-abel]

Upon cloning the project and running, the preload script located at /src/main/preload.ts does not get executed due to a typo in this section of main.ts, which is responsible for specifying the path of the file to preload. It should be preload.ts, not preload.js.

[xupea]

I don't think it will work when it's packaged since there is no .ts files at all.