[12.x] Add @includeIsolated directive for isolated Blade includes

[KennedyTedesco]

This PR introduces a new @includeIsolated Blade directive that includes a view without inheriting the parent scope's variables.

I work in a codebase where we use <x-component> for reusable components and @component() for partials (because we like variable isolation). However, @component() brings overhead we don't need: slot management, the opening/closing tag ceremony, and component lifecycle logic.

@includeIsolated would give us the best of both worlds: the simplicity of @include with the isolation of @component.

This PR is another shot of: #45616

Motivation

Currently, @include passes all parent scope variables via get_defined_vars():

{{-- Parent view has $user, $errors, $config, etc. --}}

@include('partials.hero', ['message' => $error])
{{-- hero.blade.php receives $message + ALL parent variables --}}

This implicit inheritance can lead to:

• Variable collisions in nested partials
• Unpredictable behavior when partials accidentally depend on the parent scope
• Harder debugging when variables "magically" appear

The recommended workaround is Blade components (<x-partials.hero />), but components come with overhead
(slots, validation, class resolution) that isn't always needed for simple partials.

Proposed Solution

@includeIsolated() provides explicit-only data passing where "explicit > implicit":

@includeIsolated('partials.hero', ['message' => $error])
{{-- hero.blade.php receives ONLY $message --}}

Benefits:

• Cleaner syntax for isolated partials (no closing tag needed)
• Lighter than @component (no slot/component machinery)
• Consistent with existing @includeIf, @includeWhen, @includeFirst patterns
• Prevents accidental variable leakage and naming collisions

[alies-dev]

this is great, @KennedyTedesco!!! Thank you so much!