feat: add gap cursor for top-level block nodes in default editor

[LIlGG]

What type of PR is this?

/kind feature
/area editor
/milestone 2.17.x

What this PR does / why we need it:

目前想对块级节点进行换行是一件比较困难的事情，尤其是两个相邻的块级节点之间的想插入额外的一行时更加困难。间隙光标可以解决这一问题。

为默认编辑器的顶级块节点（pos.depth = 1）增加间隙光标的功能。当 NodeType 属性 allowGapCursor 为 true 时，将会在目标节点上启用间隙光标的功能。间隙光标将可能出现在目标节点的左上方与右下方。

已知问题：

1. 对于 inlineContent 的节点，点击生成间隙光标时，光标会先出现在对应的内容上，会出现闪动的问题。
2. 在间隙光标上使用组合输入（例如中文输入）时，首个字母会被新增至新的一行。
3. CodeBlock 无法使用间隙光标（CodeBlock 自身问题，待适配）
   4. 首行空文本无法被删除（与 Gap Cursor 问题无关，待适配 Paragraph）
   5. 删除文本上方有可以添加间隙光标的块级节点时，无法触发间隙光标的 Backspace 事件（同 4，属于 Paragraph 适配问题）

目前已经启用此功能的节点：

• 表格
• 分栏卡片
• CodeBlock （无法生效）

How to test it?

测试间隙光标是否能够在表格与分栏卡片上出现。
测试间隙光标出现后，输入文本、使用快捷键等操作是否符合逻辑。
测试使用方向键调整间隙光标位置。

Does this PR introduce a user-facing change?

为默认编辑器的块级节点增加间隙光标的功能。

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 55.04%. Comparing base (5fdf6c0) to head (7d3e16b).
Report is 273 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #6103      +/-   ##
============================================
- Coverage     56.91%   55.04%   -1.88%     
- Complexity     3319     3490     +171     
============================================
  Files           587      635      +48     
  Lines         18968    21388    +2420     
  Branches       1401     1496      +95     
============================================
+ Hits          10795    11772     +977     
- Misses         7594     9010    +1416     
- Partials        579      606      +27     

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[JohnNiang]

/lgtm

[f2c-ci-robot]

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