Replace nette/utils and webmozart/assert with entropy and native PHP

[TomasVotruba]

Drops nette/utils, webmozart/assert and nette/neon as direct dependencies, replacing their usage with entropy/entropy and native PHP.

• Nette\Utils\FileSystem::read and the duplicate in-house FileSystem\FileSystem -> Entropy\Utils\FileSystem::read (3 sites)
• Nette\Utils\Strings::match -> native preg_match (9 sites)
• Nette\Utils\Strings::after -> native substr + strrpos (4 sites)
• Webmozart\Assert\Assert calls -> native guards (3 files)
• Removed nette/neon and the lint-neon script + CI step (neon-lint binary came from it)
• Test fixtures that referenced nette only as sample classes switched to native equivalents

This is step 1 of 2; a follow-up lowers the minimum PHP to 7.4.