Self-host Python bootstrap [on Windows]

As I've been setting up a new Windows machine, I was installing emsdk and found it odd that:

1. In order to run `./emsdk.ps1 install latest`, you need to have Python system-wide.
2. You install Python.
3. Once you run `./emsdk.ps1 install latest`, it downloads its own Python too.

While Python is built-in on Unix systems, it's not on Windows, and I wonder if we could make the bootstrap process easier by moving Python download step to `emsdk.ps1` (PowerShell) itself.

Then, user could just run `./emsdk.ps1 install latest`, it would download the Python locally, and then use it to run the main `emsdk.py` script.