refactor: event modernization and optimization

Summary

The motivation for this PR is to remove/replace event handling code no longer needed for legacy browsers and to optimize event handling so that we can more easily address the many scroll-related issues for the upcoming v5 release or in a future release following v5.

No features or tests have been added or removed. All of the pre-existing tests continue to pass.

Updates to Docsify's event handling are as follows:

Replace code no longer required for legacy browser compatibility with modern, native web APIs
Remove dependencies no longer required for legacy browser compatibility
Optimize performance by utilizing modern, native web APIs to reduce/remove reflows, repaints, manual calculations, and successive/unnecessary function calls (e.g., IntersectionObserver instead of scroll events)
Simplify event-related codebase by improving function names, using a consistent naming convention, adding JSDoc-style comments, and isolating function tasks and side effects.

Included but not directly event-handling related:

Removed unused SSR code
Minor cleanup of NPM scripts

NOTES:

Almost all of the event-related changes are found in /src/core/event/index.js. Other file updates are either small changes to accommodate the updates to /src/core/event/index.js or are unrelated build-related changes.
If I could have isolated the event-handling changes into separate smaller PRs, I would have. The pre-existing event handling was intertwined between multiple files and poorly named methods, making it far easier to fix in a single PR.

Related issue, if any:

None

What kind of change does this PR introduce?

Bugfix Code style update Refactor

For any code change,

N/A

Does this PR introduce a breaking change?

Tested in the following browsers:

[x] Chrome
[x] Firefox
[x] Safari

docsifyjs / docsify