pw_build: Rename pw_python_script to pw_python_action

- Rename pw_python_script to pw_python_action, which more clearly
  describes the intent of the template (wrapper around action) and
  avoids confusion with the new pw_python_package template.
- Make pw_python_script as an alias for pw_python_action.
- Update the documentation and comments, and do some minor cleanup.
- Create python_script.gni file for backwards compatibility.

Change-Id: I98ab99021c57b1357c18ec23b50c59272c61c7b9
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/22001
Reviewed-by: Alexei Frolov <frolv@google.com>
diff --git a/pw_build/exec.gni b/pw_build/exec.gni
index d4cec4f..d2b4e69 100644
--- a/pw_build/exec.gni
+++ b/pw_build/exec.gni
@@ -108,7 +108,7 @@
     _script_args += invoker.args
   }
 
-  pw_python_script(target_name) {
+  pw_python_action(target_name) {
     script = "$dir_pw_build/py/pw_build/exec.py"
     args = _script_args