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 #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 :warning:
ast/buffer.go	86.41%	5 Missing and 6 partials :warning:
ast/iterator.go	0.00%	0 Missing and 1 partial :warning:

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Additional details and impacted files

```diff @@ 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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