Implement merge feature in Tree.Edit

[hackerwins]

What this PR does / why we need it?

Implement merge feature in Tree.Edit

In a text-based editor, when you have a selection that spans two paragraphs, if you press the delete key, removes the second paragraph and merges its children into the first paragraph.

For example:

• Before: <p>a|b</p><p>c|d</p>
• After: <p>ad</p>

This commit implements the merge to Tree.Edit.

Any background context you want to provide?

// TODO(hackerwins): Define more clearly mergeable rules between fromParent
// and toParent. For now, if fromParent and toParent are the same
// type, then we can merge them.

What are the relevant tickets?

Address yorkie-team/yorkie#662

Checklist

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

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (dd9be29) 67.92% compared to head (e571547) 67.86%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #681      +/-   ##
==========================================
- Coverage   67.92%   67.86%   -0.07%     
==========================================
  Files          58       58              
  Lines        8737     8747      +10     
  Branches      785      788       +3     
==========================================
+ Hits         5935     5936       +1     
- Misses       2545     2551       +6     
- Partials      257      260       +3     

Files	Coverage Δ
src/document/crdt/tree.ts	84.58% <81.81%> (-0.17%)	⬇️

... and 1 file with indirect coverage changes

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