pref: optimize code block backspace shortcut key logic

[LIlGG]

What type of PR is this?

/kind improvement
/area editor
/milestone 2.16.x

What this PR does / why we need it:

优化默认编辑器代码块 Backspace 快捷键逻辑。如下所示：

1. 选中内容时进行删除，保证光标仍处于代码块内。
2. 光标处于代码块首位或者代码块没有内容时，不再删除代码块。

How to test it?

测试默认编辑器代码块功能。查看是否符合上述逻辑

Which issue(s) this PR fixes:

Fixes #5927

Does this PR introduce a user-facing change?

优化默认编辑器代码块 Backspace 快捷键逻辑

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 56.83%. Comparing base (5fdf6c0) to head (9293a25).
Report is 183 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #5936      +/-   ##
============================================
- Coverage     56.91%   56.83%   -0.08%     
- Complexity     3319     3498     +179     
============================================
  Files           587      612      +25     
  Lines         18968    20555    +1587     
  Branches       1401     1420      +19     
============================================
+ Hits          10795    11682     +887     
- Misses         7594     8293     +699     
- Partials        579      580       +1     

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[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.2% Duplication on New Code

See analysis details on SonarCloud

[JohnNiang]

It works very well.

/approve

[f2c-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: JohnNiang

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• ui/OWNERS [JohnNiang]

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[guqing]

/lgtm