Fix launch configs for 1.39.0 and earlier versions

[jfaltermeier]

Because of the move of electron-main from frontend to backendwe have to adjust the launch configurations as well.

Tests

mkdir my-extension-38 && cd my-extension-38
yo theia-extension -t 1.38.0
(Hello World extension type)
(hello-world extension name)
yarn build:electron
Use "Start Electron Backend" launch config

mkdir my-extension-39 && cd my-extension-39
yo theia-extension -t 1.39.0
(Hello World extension type)
(hello-world extension name)
yarn build:electron
Use "Start Electron Backend" launch config

mkdir my-extension-latest && cd my-extension-latest
yo theia-extension
(Hello World extension type)
(hello-world extension name)
yarn build:electron
Use "Start Electron Backend" launch config

[JonasHelming]

LGTM!

[vince-fugnitto]

@jfaltermeier @JonasHelming did we want to publish a new version with the fix?

[jfaltermeier]

I guess it would be nice to have a fixed version. I believe the default version is latest, and this did not work.