Enhance entry.py to allow for an additional package.json

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**Is your feature request related to a problem? Please describe.**
It would be useful for [robot-framework-browser-extensions](https://github.com/MarketSquare/robotframework-browser-extensions) if `rfbrowser init` would allow for an additional path to a package.json used by the browser extensions. That would make that the node packages will all live in the same location, which I think is preferable. It would also ease the use of extensions

**Describe the solution you'd like**
I'd like something like
```
def _rfbrowser_init(skip_browser_install: bool, file: str):
```
usage:
`rfbrowser init skip_browser_install file="path/to/extensions/package.json"`

**Describe alternatives you've considered**
Alternatives are folks having to manually install and debug the package installation themselves and you're likely to end up with multiple `node-modules` folders in your project, which I consider undesirable.

Start in entry.py and trace the implementation behind `rfbrowser init`, including how its existing package installation path is selected. Confirm how an additional package.json path should be accepted and verify that the existing initialization behavior remains intact while packages use the intended shared location.

Written by the indexing model from the issue text.