pw_watch: Rework arguments

- Update the command line arguments to make pw watch simpler to use.
  Positional arguments are interpreted as targets in the default output
  directory. -C or --build_directory can be provided to support
  specifying the build directory or using multiple directories.
- Find Ninja directories by build.ninja instead of args.gn to support
  CMake.
- Call .kill() instead of .terminate() on Ninja to give it time to
  properly clean up before closing.
- Import Path from pathlib, for consistency with other modules.

Change-Id: Ic66e58ce775de4a3a8749109d4ff65eb78a22aac
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/25760
Reviewed-by: Keir Mierle <keir@google.com>
Commit-Queue: Wyatt Hepler <hepler@google.com>
diff --git a/pw_build/docs.rst b/pw_build/docs.rst
index d1c5b46..cd78693 100644
--- a/pw_build/docs.rst
+++ b/pw_build/docs.rst
@@ -391,7 +391,7 @@
 
 .. code-block:: sh
 
-  pw watch out/cmake_host pw_run_tests.modules
+  pw watch -C out/cmake_host pw_run_tests.modules
 
 CMake functions
 ---------------