Introduce TreeToken and tokensBetween to Tree

[sejongk]

What this PR does / why we need it:

Unlike JS SDK, this PR doesn't create TreeChanges, but it changes the traversal logic to be token-based to keep consistency.

• Implements token traversal method.
• Ensures accurate inclusion of tokens (Start/End/Text) in ranges.

Which issue(s) this PR fixes:

Addresses #732

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Additional documentation:

Checklist:

• Added relevant tests or not required
• Didn't break anything

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (64cbc24) 49.27% compared to head (cae18dc) 49.30%.

Files	Patch %	Lines
pkg/document/crdt/tree.go	85.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #747      +/-   ##
==========================================
+ Coverage   49.27%   49.30%   +0.03%     
==========================================
  Files          69       69              
  Lines       10087    10094       +7     
==========================================
+ Hits         4970     4977       +7     
  Misses       4601     4601              
  Partials      516      516              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[hackerwins]

Thanks for your contribution.