Naming `BrowserPlugin` vs `BrowserFactory`

[vdusek]

• Naming browsers/browser_plugin.py vs browsers/browser_factory.py (or BrowserControllerFactory).
• "Plugin" is the old name and doesn't quite fit the current use case. "Factory," on the other hand, seems to be a better fit, as the class has only one public method for producing BrowserControllers, along with initialization and destruction. However, we would need to update this in the TypeScript version as well.
• This is a follow-up to the issue discussed in this pull request comment.