GHSA-r557-wffq-wvrc

### Impact

With `trailingSlash: 'always'` configured, the `@astrojs/node` standalone server's static file handler appends a trailing slash to request paths and issues a `301` redirect. Paths beginning with `/\` (slash-backslash) were not recognized as internal paths, so the handler would echo the raw path back in the `Location` header. Because browsers treat `\` as `/` per the WHATWG URL specification, the resulting redirect could resolve to an external host.

**Preconditions:** - `trailingSlash: 'always'` must be set (non-default; the default is `'ignore'`) - The request path must not have a file extension in its final segment - An attacker must deliver the crafted link to a user

### Patches

Fixed by treating backslash-prefixed paths the same as `//`-prefixed paths in `isInternalPath()`, so they are no longer rewritten with a trailing slash.

### Workarounds

Use the default `trailingSlash: 'ignore'` setting, which does not issue trailing-slash redirects in the static file handler.

### References

- [WHATWG URL spec: backslash normalization](https://url.spec.whatwg.org/#url-parsing)