ref(nextjs): Simplify NextConfigObject type

[lobsterkatie]

This simplifies the NextConfigObject type in the nextjs SDK, to enable future work. Key changes:

• Removed index signature.
• Added missing basePath and publicRuntimeConfig properties.
• Made all properties optional. (This allowed for removal of the Partial annotation everywhere the type was used.)
• Clarified variable names in config/index.ts to better reflect their types.

[FranciscoKloganB]

The removal of lines 21 through 23 breaks my build.

 & {
  // other `next.config.js` options
  [key: string]: unknown;
};

> tsc --project tsconfig.json

next.config.js:43:24 - error TS2345: 

    Argument of type 
        '{ 
            compiler: { styledComponents: boolean; };
            i18n: { locales: string[]; defaultLocale: string; localeDetection: boolean; };
            images: { domains: string[]; };
            reactStrictMode: boolean; 
            swcMinify: boolean; 
        }' 

is not assignable to parameter of type 'ExportedNextConfig | undefined'.

43  ? withSentryConfig(nextConfig, sentryWebpackPluginOptions)
                          ~~~~~~~~~~