fix:(ast) implement soft delete for Unset()

[AsterDY]

Background

• Before v1.9.2, ast.Node.Unset() will hard delete child for one node. But since refactor:(ast) use linked chunk as fundamental storage for nodes #464 introduced llnked-chunk as storage, the behavior changed: Unset() will set an empty node at the index but still count it. This inconsistency disturbs our users, although current behavior sounds more reasonable asUnset(). Thus, we decide to be compatible with the old behavior.
• Besides, UnsetByIndex() is not a behavior provided by std slice, we recommend users to use Add() and Pop() instead

Feature

• Len() and Index() won't count emtpy nodes, as implemenation of soft-delete, which comes with overhead of index query O(N)
• Add Pop(), Move() API as slice-like OP

[codecov-commenter]

Codecov Report

Attention: 35 lines in your changes are missing coverage. Please review.

Comparison is base (ec2ab23) 78.57% compared to head (02ffc26) 78.57%.

Files	Patch %	Lines
ast/node.go	70.88%	14 Missing and 9 partials ⚠️
ast/buffer.go	86.41%	5 Missing and 6 partials ⚠️
ast/iterator.go	0.00%	0 Missing and 1 partial ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #571      +/-   ##
==========================================
- Coverage   78.57%   78.57%   -0.01%     
==========================================
  Files          69       69              
  Lines       10716    10823     +107     
==========================================
+ Hits         8420     8504      +84     
- Misses       1930     1942      +12     
- Partials      366      377      +11     

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