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 ~55ms, out of which getting response from
watchman daemon takes ~35ms, so there is room for further improvements.

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: #261