Sets up the webpack configuration to use tsconfig-paths-webpack-plugin, which will
add aliases for webpack that will match what is found in the tsconfig.json.
The plugin options defined by the user
These are the lists of extensions for this plugin to process
Currently, it is processing all typescript and javascript,
since that is assumed to be how typescript will be loaded by
Gatsby.
The extensions option of of this plugin should match the general
webpack resolution.
['.ts', '.tsx', '.js', '.jsx']Array of valid file extensions
Same interface that gatsby uses for the
onCreateWebpackConfig()function