Warn on implicit switch case fallthrough

[lf-]

This seems to have found one actual bug in fs-sink.cc: the symlink case was falling into the regular file case, which can't possibly be intentional, right?

Motivation

Trivially easy to do, catches bugs, doesn't degrade dev experience (just use [[fallthrough]], // fall through, etc to disable it if it's deliberate).

Context

https://docs.jade.fyi/gnu/gcc/gcc.html#index-Wimplicit_002dfallthrough_003d

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