fix(gatsby-plugin-nginx): Spread routes support

[tlgimenes]

What's the purpose of this pull request?

This PR adds support to spread routes only.

How it works?

Suppose we have the following taxonomy:

pages
...
│
├── {StoreCollection.slug}
│   └── [...].tsx
...

Currently, any page created will be available at, and only at: /:slug/. If the user enters in /:slug it will receive a 404.
This PR solves this problem by making the route /{StoreCollection.slug}/ also respond at /:slug.

Note that if the user has the following taxonomy:

├── account.tsx
├── {StoreCollection.slug}
│   └── [...].tsx
│   └── index.tsx
...

Both account.tsx and index.tsx will still work because of route precedence.

How to test it?

Make sure no route was lost on either of these stores:

https://github.com/vtex-sites/storecomponents.store/pull/1124
https://github.com/vtex-sites/marinbrasil.store/pull/610
https://github.com/vtex-sites/btglobal.store/pull/789

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[cezar-guimaraes]

LGTM