Add code block copy button (#433)

[yzxh24]

Added a copy button to the top right corner of the code block, click to copy all the code, like this:

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 70.42254% with 21 lines in your changes missing coverage. Please review.

Project coverage is 87.93%. Comparing base (c391628) to head (e22e516).

Files with missing lines	Patch %	Lines
packages/fleather/lib/src/widgets/code_color.dart	60.86%	9 Missing :warning:
.../fleather/lib/src/widgets/editable_text_block.dart	41.66%	7 Missing :warning:
...fleather/lib/src/rendering/editable_text_line.dart	82.14%	5 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #434      +/-   ##
==========================================
- Coverage   88.04%   87.93%   -0.11%     
==========================================
  Files          62       63       +1     
  Lines       10419    10486      +67     
==========================================
+ Hits         9173     9221      +48     
- Misses       1246     1265      +19     

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[amantoux]

@yzxh24 Thank you for the PR Let us know when it is ready to be reviewed