feat: on page load handler

[jhutchins]

What kind of change does this PR introduce?

• Bugfix
• Feature
• Docs
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change?

• Yes
• No

Checklist

• This PR will resolve #___
• A change file is added if any packages will require a version bump due to this PR per the instructions in the readme.
• I have added a convincing reason for adding this feature, if necessary
• It can be built on all targets and pass CI/CD.

Other information

[jhutchins]

I haven't added the change files yet and I need to test the Mac implementation, which I'm the most uncertain about, but thought I'd put this out there for early feedback.

[jhutchins]

I've added all three handlers and I believe addressed all concerns and comments.

[jhutchins]

I'm just going to remove the handler for navigation starting.

[amrbashir]

LGTM but I will keep the PR open a while for @wusyong and @lucasfernog to give their approvals too

[wusyong]

LGTM but there's still a clippy error.

[lucasfernog]

I would consider using a single event function with_on_page_load_handler with an enum payload PageLoadEvent { Started, Finished } instead of two separate functions.

[jhutchins]

@wusyong There is a clippy error, but I'm not sure how you all would like it resolved. The error is due to new lints applied to old code, not anything that I added. The issue is a structure that is a unit class for some environment and not for others so default is used to create it, but using default to build a unit struct is now a clippy violation. I think the most straight forward is to add a lint exclusion, but it's possible you want a different solution.

@lucasfernog I could make that change.

[jhutchins]

@lucasfernog I made those changes to the public API.

@wusyong I opted to just just allow the clippy violation, but could do something else.