Allow multiple proxies for dev server.

[hayleigh-dot-dev]

Currently, users can configure an api proxy for the development server by adding the following to their gleam.toml:

[tools.lustre.dev]
proxy = { from = "/api", to = "http://localhost:3000/api" }

It's not uncommon to want to configure multiple proxies, for example to serve additional static assets from a separate server app. We should support both single object proxy configuration and array of objects!