set default pageview "props" to the tracker

[mouhannad-sh]

It would be nice if we can set default props (context) to be sent with every pageview event. So that we can use enableAutoPageviews()

Detailed Description

Right now we can only send custom props with pageviews events using the trackPageview() function.
As a customer who's a multi-tenancy platform provider, I benefit from the simple implementation of the tracker by using enableAutoPageviews(), but if we want to set custom props (e.g. subscription-plan) we'll need to opt-out from using enableAutoPageviews and write our own logic to track every pageview with the desired props

Possible Implementation

I can think of 2 implementations that might work here

1- Allow EventOptions to be passed to enableAutoPageviews

While this might be the simplest to implement, I'm not a big fan of this approach as it would make updating the eventOptions a bit challenging.

PlausibleInstance.enableAutoPageviews(eventOptions?: EventOptions)

2- Add a pageviewProps option to Plausible definition and setPageviewProps to the plausible instance

I'm in favor with this approach as it's easy to use and can be introduced without any breaking changes

const tracker = Plausible({
  domain: "plausible.io",
  pageviewProps: { isLoggedIn: "No" }
});

// auto track pageviews
tracker.enableAutoPageviews()

// and then to update it when needed
tracker.setPageviewProps({ subscriptionPlan: "Enterprise", isLoggedIn: "Yes", ...others })

I'm happy to open a PR with either approaches if needed

[Barbapapazes]

Hello,

Since this package have not been update for two years, I made a fork.

I rewrite the tracker to be more modular and I fix this issue.

https://github.com/Barbapapazes/plausible-tracker