🐛 BUG: Inconsistent behaviour in `_routes.json`. Also, no way to specify index file.

### What version of `Wrangler` are you using?

2.1.12

### What operating system are you using?

Windows

### Describe the Bug

When using a `_routes.json`, the following:
```json
"include": [
"/*"
],
"exclude": [
"/"
]
```
–results in all paths being excluded.

What I expected to happen was `/` (index.html) would be excluded, while everything else was included. I imagine this isn't intentional, as this sounds like what should happen if both include and exclude were set to `/*`, except that Wrangler won't let you do that.

That being said, I cannot find a way to reference a root level index file, as `/` doesn't work.

I tested this both on Wrangler and deployed; the behaviour is the same.