Implement splitLevel of Tree.Edit

[hackerwins]

What this PR does / why we need it:

When a user positions the cursor within a paragraph and hits the enter key, the paragraph divides into two separate paragraphs:

• Original: <p>a|b</p>
• Result: <p>a</p><p>b</p>

In this commit, splitLevel is introduced to support this scenario.

doc.update((root) => {
  root.tree.edit([0,0], {type:'p',children:[{type:'text', value: 'ab'}]});
  root.tree.edit([2,2], undefined, 1); // split the paragraph
});

This commit does not cover all test cases that may arise due to the split operation. This is because we need to verify the interface first. we need to add test cases in the simultaneous editing situation to verify the algorithm later.

Tasks:

• Refactor Tree.Edit logic to implement splitLevel of Tree.Edit
• Remove skip from the unit tests related to splitLevel

Which issue(s) this PR fixes:

Address #662

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: 75 lines in your changes are missing coverage. Please review.

Comparison is base (12e86a2) 48.81% compared to head (c76e233) 49.11%.

Files	Patch %	Lines
pkg/document/crdt/tree.go	71.16%	43 Missing and 19 partials ⚠️
pkg/index/tree.go	18.18%	9 Missing ⚠️
api/converter/from_pb.go	0.00%	2 Missing ⚠️
api/converter/to_pb.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #705      +/-   ##
==========================================
+ Coverage   48.81%   49.11%   +0.29%     
==========================================
  Files          69       69              
  Lines       10127    10194      +67     
==========================================
+ Hits         4944     5007      +63     
+ Misses       4657     4656       -1     
- Partials      526      531       +5     

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