feat: Add support for using watchman(1) instead of walking filesystem

Anecodically, for project having 4k files and 7 different formatters,
this change can reduce time it takes for treefmt(1) to figure out that
it has nothing to do from ~350ms down to ~8m.

If "WATCHMAN_SOCK" environment variable is not set and "watchman(1)"
command are not available, "treefmt" transparently falls back on walking
the file system. Setting the environment variable is highly recommended,
since is avoids subprocess invocation.

Ref: numtide#261