Allow Vite entry points to be merged

[jackwh]

This PR aims to solve an issue when Vite entry points are conditionally determined at runtime.

The existing withEntryPoints method in the Vite class replaces any entry points which have already been applied, and $this->entryPoints is protected with no accessor method. So this backwards-compatible PR simply adds an additional method to merge a set of entry points with any which have already been set.

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I recently found myself in this situation:

• I have two main Vite entry points, app.css and app.js.
• I also have some additional entry points for custom React components: component-1.ts, component-2.ts etc.
• My app uses Filament for the backend admin functionality, so I've used ->viteTheme(['app.css', 'app.js']) to set my entry points in their service provider.
• On pages which require a specific React component, I can add @vite(['component-2.ts']) to include them as needed.

However, if component-2.ts references assets which are also used by app.css (e.g. website logo etc.), duplicate chunk tags are rendered in the HTML markup.

With this PR I can let Filament call withEntryPoints in its service provider, then later attach any additional components I need to include conditionally, before letting Filament render them once in a single output.

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There are other ways to solve this, I figured mergeEntryPoints was the simplest but am open to any other ideas.

Let me know if any changes are necessary. Thanks! 🙏