feat(router): add 'current_branch' router

[linrongbin16]

Regression Test

Platforms

• windows
• macOS
• linux

Hosts

• Test on github.com.
• Test on gitlab.com.
• Test on bitbucket.org.
• Test on codeberg.org.
• Test on git.samba.org.

Functions

• Use GitLink(!) to copy git link (or open in browser).
• Use GitLink(!) blame to copy the /blame link (or open in browser).
• Use GitLink(!) default_branch to open the /main//master link in browser (or open in browser).
• Copy git link in a symlink directory of git repo.
• Copy git link in an un-pushed git branch, and receive an expected error.
• Copy git link in a pushed git branch but edited file, and receive a warning says the git link could be wrong.

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 76.18%. Comparing base (944a4b8) to head (efe2e80).

❗ Current head efe2e80 differs from pull request most recent head e230ee9. Consider uploading reports for the commit e230ee9 to get more accurate results

Files	Patch %	Lines
lua/gitlinker/configs.lua	85.71%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #220      +/-   ##
==========================================
+ Coverage   75.61%   76.18%   +0.56%     
==========================================
  Files           9        9              
  Lines         767      802      +35     
==========================================
+ Hits          580      611      +31     
- Misses        187      191       +4     

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