feat: add runtime config support

[BobbieGoede]

• Closes #443

Merges the config found in publicRuntimeConfig.firebase with the module options, I don't think this works if you add the module to buildModules.

Adds functionality to compile/generate templates on startup, maybe there is a better way that doesn't require adding lodash and serialize-javascript as dependencies. The compilation also makes the assumption that the nuxt template global is always set to $nuxt, this can probably be cached/saved at build time for accuracy.

For dynamic configs the usage could be as follows

publicRuntimeConfig: {
    firebase:
        process.env.CUSTOM_ENV_VARIABLE === 'production' ?
        // Production config
        {
            apiKey: '...',
            authDomain: '...',
            projectId: '...',
            storageBucket: '...',
            messagingSenderId: '...',
            appId: '...',
            measurementId: '...',
        } :
        // Development config
        {
            apiKey: '...',
            authDomain: '...',
            projectId: '...',
            storageBucket: '...',
            messagingSenderId: '...',
            appId: '...',
            measurementId: '...',
        },
}

To me this seems a bit easier or more flexible than the customEnv key and the nested environment configs. I suggest removing that approach if this feature gets added as the same can be achieved with runtime config and it could cause confusion/conflicts if nested configs are used in combination with runtime config.

My implementation feels a bit hacky to me, so please let me know if there are any changes to be made! 😄