Migrate Site/index.js to TypeScript

[jovyntls]

What is the purpose of this pull request?

• Documentation update
• Bug fix
• Feature addition or enhancement
• Code maintenance
• DevOps
• Improve developer experience
• Others, please explain:

Part of #1913

Overview of changes:
Migrates Site/index.js to TS

Anything you'd like to highlight/discuss:
If we want to refactor how the methods at the end of Site/index.js are handled, I'd suggest we do a separate refactor of these methods after migrating this file to TS. Rationale of having these methods defined with Site.prototype: see #979

Testing instructions:
N/A

Proposed commit message: (wrap lines at 72 characters)
(rebase commit)

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Checklist: ☑️

• Updated the documentation for feature additions and enhancements
• Added tests for bug fixes or features
• Linked all related issues
• No unrelated changes

[raysonkoh]

Thanks for the hardwork! Added some preliminary comments.

[raysonkoh]

LGTM. Thanks @jovyntls!

[tlylt]

Thanks @jovyntls for this 💪, have taken a brief look and LGTM.

[ang-zeyu]

@tlylt as it will be quite a bit of work to undo the revert while keeping track of new PRs editing the file #2308 (comment), I suggest adding a small follow up comment on top that prohibits any changes to this file until this is fixed (and bumping the priority).

The alternative is letting a portion of the typescript migration work go to waste (also an ok approach).
I'll leave this up to you.