[FEATURE] Watch a directory recursively

**Is your feature request related to a problem? Please describe.**

Not quite. Most of my business logic lives in subdirectories but is orchestrated through a `main.py` and `config.py` in the top-level of the repo. Since tkreload only watches _one_ file or the entrypoint, it misses the child directory (module) changes.

**Describe the solution you'd like**

Hot reload using a directory or multiple files (e.g., `tkreload .`, `tkreload main.py core/ static/ utils/`). If an executable file isn't automatically detected, could prompt the user for the filename, use kwargs, or env vars.

**Describe alternatives you've considered**

Could probably hack something together using the [watchdog](https://python-watchdog.readthedocs.io/en/stable/quickstart.html?highlight=recursive#a-simple-example) library, but tkreload works great for single files as-is.

**Additional context**

N/A

Start by reading tkreload's current single-file or entrypoint watching flow, then review the linked watchdog recursive example. Reproduce the requested commands with main.py, config.py, and paths such as core/, static/, and utils/; done means changes under watched directories or multiple files trigger reloads while executable-file selection is handled.

Written by the indexing model from the issue text.