refactor(dirvish): extract cj/--dired-line-is-directory-p

`cj/dired-mark-all-visible-files' classified the current line as a directory via `(looking-at "^. d")' inline.  Lift the classification into `cj/--dired-line-is-directory-p', a string predicate that takes a line and returns non-nil when it's a directory listing.  The wrapper still walks the dired buffer line by line and calls `dired-mark' -- that iteration is dired-coupled and stays untested -- but the format-aware predicate is now isolated and verified.

Six Normal/Boundary tests cover unmarked directories, marked directories (`*' prefix), regular files (`-' instead of `d'), symlinks (`l'), empty lines, and dired header lines (`  /path:' and `  total N').

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