file-icons: accept false in files and languages to turn icons off

[ewels]

Closes #7

fileIcons.files and fileIcons.languages[lang].icon now accept false, meaning "no icon". A site can turn icons off for one file type or language (such as .mmd files) without icon=false on every block.

• A false match stops the search: it does not fall through to the language or the generic file icon.
• files keys keep their existing precedence (the later key wins), so { '.mmd': false, 'overview.mmd': 'markdown' } still gives that one file an icon.
• icon="<name>" on the fence line still wins over both, and icon=false / no-icon are unchanged.
• The code tabs menu shows its generic code icon for a language with icon: false.

Tests in test/file-icons.test.ts cover the cases above, the defaults, and validation (accepts false, still rejects true). The docs page, option descriptions and the skill are updated.

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