Warn if `files` doesn't resolve to a file

`files` leaves room for subtle mistakes that are hard to catch at review time, such as:
```js
"files": [
"vite.config.js", // maybe the actual extension is ts
"../../vite.config.common.ts" // maybe there's a wrong number of ../s
```

If a literal path is specified that doesn't match an actual file, Wireit should throw an error.

See also #215