Code review: isSubdir

[cscheid]

We currently vendor isSubdir from Deno's stdlib, and their documentation (or code, take your pick) describes parameters src and dest precisely backward. We use this call in Quarto ourselves: https://github.com/quarto-dev/quarto-cli/blob/main/src/deno_ral/fs.ts#L49. We need to review all callers of this function for accuracy, and then fix the function code or docs.