feat: trim whitespace from concise mode fences

[LuLaValva]

In previous versions of this parser, concise mode code fences preserved their leading and trailing whitespace

--
Hello World
--
// `\nHello World\n`

These changes trim that whitespace, and also whitespace from leading tabs

[changeset-bot[bot]]

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Latest commit: 367f4e6f67fda736a43ededc199c4dc103969d55

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