identifier is always lower!!

[GabBiswajit]

Summary: This patch fixes a texture loading issue caused by uppercase letters in custom item/block identifiers. Details:

• Converts $identifier to lowercase during registration.
• Ensures consistent identifier formatting without modifying texture filenames or resource pack contents.
• Prevents case-sensitive mismatches between game engine and resource definitions. Impact: Improves reliability of texture loading for custom items/blocks by enforcing lowercase identifiers. No changes to texture files or resource pack structure required.