Add constants for page resource schema slots

[Voltra]

To avoid hard-coding strings in resource classes, typos, and for better autocompletion and "future-proofing", this PR introduces a new class Z3d0X\FilamentFabricator\View\ResourceSchemaSlot similar to #193 for layout render hooks

This means that codebases can migrate resource classes from:

FilamentFabricator::getSchemaSlot('sidebar.after')

to

FilamentFabricator::getSchemaSlot(\Z3d0X\FilamentFabricator\View\ResourceSchemaSlot::SIDEBAR_AFTER)

or even

use Z3d0X\FilamentFabricator\View\ResourceSchemaSlot;

// [...]

FilamentFabricator::getSchemaSlot(ResourceSchemaSlot::SIDEBAR_AFTER)

[what-the-diff]

PR Summary

• Modification in PageResource.php
  The PageResource.php file has been updated to reference constants from a new class called ResourceSchemaSlot rather than using direct string literals. This change enhances code robustness ensuring fewer errors due to potential string literals' misspellings.

• Introduction of ResourceSchemaSlot.php
  A fresh file named ResourceSchemaSlot.php has been incorporated. It introduces constants for various schema slots like BLOCKS_BEFORE, BLOCKS_AFTER, SIDEBAR_BEFORE, and SIDEBAR_AFTER. This addition promotes code readability and maintainability by minimizing the use of raw strings.

[Z3d0X]

I'm still kind of leaning towards this.
#94 (comment)

[Voltra]

They're both compatible :)