Add `PluginGroupBuilder::replace` helper method

## What problem does this solve or what need does it fill?

Bevy is meant to be modular, which means adding, disabling, and swapping out plugins. However, some ergonomics are missing for _swapping_ when using plugin groups.

## What solution would you like?

```rust
DefaultPlugins
.build()
.replace::(OtherAssetPlugin { ... })
.finish()
```

## What alternative(s) have you considered?

```rust
DefaultPlugins
.build()
.disable::()
.add_after::(OtherAssetPlugin { ... })
.finish()
```

Start by locating PluginGroupBuilder and reading the existing disable, add_after, and finish methods. Check how plugin groups track ordering and how related behavior is tested. Done means replace::(...) swaps the matching plugin and preserves the expected group behavior, with the relevant tests passing.

Written by the indexing model from the issue text.