feat: nextrouter pkg to handle nextjs routing rules (#167)

An issue came up last week... our `embed.go` strategy doesn't handle dynamic NextJS-style routes! This is a blocker, because I'm aiming to set up CD on Monday, and the v2 UI makes heavy use of dynamic routing.

As a potential solution, this implements a go pkg `nextrouter` that serves `html` files, but respecting the dynamic routing behavior of NextJS:
- Files that have square brackets - ie `[providers]` provide a single-level dynamic route
- Files that have `[[...` prefix - ie `[[...any]]` - are catch-all routes.
- Files should be preferred over folders (ie, `providers.html` is preferred over `/providers`)
- Fixes the trailing-slash bug we hit in the previous `embed` strategy

This also integrates with `slog.Logger` for tracing, and handles injecting template parameters - a feature we need in v1 and v2 to be able to inject stuff like CSRF tokens.

This implements testing by using an in-memory file-system, so that we can exercise all of these cases.

In addition, this adjust V2's `embed.go` strategy to use `nextrouter`, which simplifies that file considerably. I'm tempted to factor out the `secureheaders` logic into a separate package, too.

If this works OK, it could be used for V1 too (although that scenario is more complex due to our hybrid-routing strategy). Based on our FE variety meeting, there's always a chance we could move away from NextJS in v1 - if that's the case, this router will still work and be more tested than our previous strategy (it just won't make use of dynamic routing). So I figured this was worth doing to make sure we can make forward progress in V2.

coderd/coderd.go

@@ -126,7 +126,7 @@ func New(options *Options) http.Handler {
 			})
 		})
 	})
-	r.NotFound(site.Handler().ServeHTTP)
+	r.NotFound(site.Handler(options.Logger).ServeHTTP)
 	return r
 }