feat(core): convert the gist api handler to TypeScript

[marcalexiei]

Another step of #140

• api/gist.js ➡️ api/gist.ts
• The handler parses (parseFloat, parseBoolean); the card still defaults.
  Passing undefined for an absent param keeps every default in one place.
• CommonOptions ➡️ CommonCardOptions, now extending ColorParams since cards spread
  their options into getLightDarkColors.
• locale moved onto the four cards that read it: the gist card has no translated text.
  The gist handler no longer takes or validates it, so /api/gist?locale=xx renders the card instead of "Language not found". Docs has been updated.
• border_radius is now validated at the boundary: ?border_radius=abc returns a permanent Invalid number input for parameter "border_radius" instead of a temporary error leaking Card's internal Invalid border radius: "NaN".

[vercel]

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