fix: detect active and exact active link

[farnabaz]

Types of changes

• Bug fix (a non-breaking change which fixes an issue)
• New feature (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Description

Refactor active link detection of TOC and user scroll listener instead of InsersectionObserver. resolves #63

A link will mark as active if:

• It become visible in upper half of page
• It has a active child

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes (if not applicable, please state why)

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