feat!: add output formatter

[neilime]

Separate output formatting in a dedicated domain

• Allows to specify formatter(s) via cli argument
• Supports following formatters:
  • plain (default original one)
  • Github Actions
  • Gitlab CI

[codecov]

Codecov Report

Attention: Patch coverage is 96.95946% with 9 lines in your changes missing coverage. Please review.

Project coverage is 96.00%. Comparing base (6f1b011) to head (93f1950).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
src/CssLint/Cli.php	94.73%	4 Missing ⚠️
src/CssLint/Output/FileOutput.php	86.66%	2 Missing ⚠️
...ssLint/Output/Formatter/GithubActionsFormatter.php	96.29%	2 Missing ⚠️
src/CssLint/Output/Formatter/GitlabCiFormatter.php	98.64%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #124      +/-   ##
============================================
+ Coverage     95.89%   96.00%   +0.10%     
- Complexity      417      489      +72     
============================================
  Files            29       36       +7     
  Lines          1292     1526     +234     
============================================
+ Hits           1239     1465     +226     
- Misses           53       61       +8     

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