Add PHPCS and correct app/Importer/ for PSR-12 (errors only)

[CloCkWeRX]

Blocked by grokability#16767

@CloCkWeRX ➜ /workspaces/snipe-it (phpcs) $ php phpcs.phar -n --standard=PSR12 app/Importer/
FILE: /workspaces/snipe-it/app/Importer/AssetModelImporter.php
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FOUND 2 ERRORS AFFECTING 2 LINES
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 128 | ERROR | [x] There must not be a space before the colon in a return type declaration
 148 | ERROR | [x] There must not be a space before the colon in a return type declaration
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PHPCBF CAN FIX THE 2 MARKED SNIFF VIOLATIONS AUTOMATICALLY
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FILE: /workspaces/snipe-it/app/Importer/Importer.php
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FOUND 1 ERROR AFFECTING 1 LINE
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 254 | ERROR | Method name "Importer::array_smart_custom_field_fetch" is not in camel caps format
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Time: 184ms; Memory: 10MB