Shaders can now have #else ifdef chains (#7431)

# Objective

Currently, shaders may only have syntax such as

```wgsl
#ifdef FOO
// foo code
#else
#ifdef BAR
// bar code
#else
#ifdef BAZ
// baz code
#else
// fallback code
#endif
#endif
#endif
```

This is hard to read and follow.
Add a way to allow writing `#else ifdef DEFINE` to reduce the number of scopes introduced and to increase readability.

## Solution

Refactor the current preprocessing a bit and add logic to allow `#else ifdef DEFINE`.
This includes per-scope tracking of whether a branch has been accepted.
Add a few tests for this feature.

With these changes we may now write:

```wgsl
#ifdef FOO
// foo code
#else ifdef BAR
// bar code
#else ifdef BAZ
// baz code
#else
// fallback code
#endif
```

instead.

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## Changelog

- Add `#else ifdef` to shader preprocessing.