Allow specifying more specific icon theme associations

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VSCode gives icon theme developers the ability to set icons for files through keys like fileExtensions, fileNames, folderNames, as well as what Zed already offers in languageIds for file types. With Zed, only being able to set icons based on file type means for a file like flake.lock - which is recognized as JSON but also set as the lock "file type" based on its extension, matched via this hard coded suffix list: https://github.com/zed-industries/zed/blob/main/assets/icons/file_icons/file_types.json - it is not possible for me to set flake.lock to use a different icon (nix-lock) than other "lock" files (e.g. bun.lock, which would be bun-lock).

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Reference: https://code.visualstudio.com/api/extension-guides/file-icon-theme#file-association