Add simple support for dependent python modules (#11)

* add pipinstall script and append py_modules to PYTHONPATH

allows dependent python modules to be installed

* separate build_frontend and pip_install

* use prepare script for installing python modules

* use pip3 instead of pip

* always create requirements.txt, even if empty

package.json

@@ -3,6 +3,7 @@
   "version": "0.0.1",
   "description": "A template to quickly create decky plugins from scratch, based on TypeScript and webpack",
   "scripts": {
+    "prepare": "touch requirements.txt && pip3 install --upgrade -r requirements.txt -t py_modules",
     "build": "shx rm -rf dist && rollup -c",
     "watch": "rollup -c -w",
     "test": "echo \"Error: no test specified\" && exit 1"