Add documentation for highlight and fix bug

[aaachen]

This change fixes the merge highlight issue: #2996

Problem

Consecutive highlights will automatically get merged into one.

The root cause is

1. The highlight end offset is non-inclusive (highlight is only supported over text. The offset is counted by characters for all elements' text content)
2. The overlapping highlight calculation treats the end offset as inclusive (see useSelection.ts)

Changes

1. Corrected the logic
2. Added documentations
3. Correct a new visual bug resulting from changes of 1). See below section for description

Visual Bug

Example sequence:

selection cursor ends at empty area, selecting no text at the focus node

create highlight (note: the word share is not selected)

create another highlight, note the highlight overlap visual bug

removing one of highlight sometimes result in the patching to go wrong and jumble the source text#3165

The fix for this visual bug is to clip the selection to the preceding/succeeding DOM element by updating the selection range.

Testing

Manually tested

1. Forward select

2. Reverse select

[vercel]

@aaachen is attempting to deploy a commit to the omnivore Team on Vercel.

A member of the Team first needs to authorize it.

[vercel]

The latest updates on your projects. Learn more about Vercel for Git ↗︎

Name	Status	Preview	Comments	Updated (UTC)
omnivore-demo	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Jan 5, 2024 5:17am
omnivore-prod	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Jan 5, 2024 5:17am

[jacksonh]

FYI, this one looks good to me. Just fighting a couple fires so I don't want to merge it until early next week.

[jacksonh]

OK, going to get this one into our test environment finally. Sorry on the delay. Putting out some fires this week