feat(router): add 'default_branch' router type

[linrongbin16]

Resolve #151 .

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.
• Use GitLink! to open git link in browser.
• Use GitLink blame to copy the /blame git link.
• Use GitLink! blame to open the /blame git link 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 73.86%. Comparing base (f205e64) to head (5a9d207).

❗ Current head 5a9d207 differs from pull request most recent head 50b04bc. Consider uploading reports for the commit 50b04bc to get more accurate results

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

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #207      +/-   ##
==========================================
+ Coverage   72.83%   73.86%   +1.03%     
==========================================
  Files           8        8              
  Lines         692      727      +35     
==========================================
+ Hits          504      537      +33     
- Misses        188      190       +2     

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