Add reader mode for in-app browser

[micahmo]

Pull Request Description

This PR adds a reader mode for Android (and iOS) using the in-app browser. Where previously the reader mode option would only be shown on iOS, it is now shown on Android too if the in-app browser is selected. It can also be toggled on/off dynamically in the in-app browser, regardless of the default. Note that history isn't shared between the modes.

One potential downside is that the package I'm using for this hasn't been updated for a while, but after a considerable amount of searching (which took a while, since it isn't even on pub.dev), it was not only exactly what I was looking for but seems to work very well. (I would've been happy with a simple web scraper that returns the body contents for us to render, but this has a whole controller architecture and everything.)

Aside from that I think it's pretty easy to sell the usefulness of a cross-platform reader mode. It makes page loads and navigation much faster and scrolling is more responsive.

Note: I also introduced some abstraction to handle the different types of controllers so there aren't a lot of conditionals everywhere.

Review without whitespace.

Issue Being Fixed

Issue Number: #760

Screenshots / Recordings

Video was too big for GitHub.

https://files.catbox.moe/aetnb4.mp4

Checklist

• Did you update CHANGELOG.md?
• Did you use localized strings where applicable?
• Did you add semanticLabels where applicable for accessibility?

[hjiangsu]

Hey! Finally getting around to this one - my only worry with this is that the package will be come outdated as time goes on. We can create a fork of this on the thunder repo, but it most likely means we'll have to maintain it. Do you have any thoughts on this?

[micahmo]

It's totally your call if you don't mind taking on another package to maintain. Of course I'm willing to help as needed! But I do think this is a pretty neat feature to have to flesh out in the in-app browser, and this package fits in perfectly with our existing browser controller architecture.