feat(router): add 'default_branch' router type

[linrongbin16]

Resolve #151 .

Regression Test

Platforms

• [ ] windows
• [x] macOS
• [ ] linux

Hosts

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

Functions

• [x] Use GitLink to copy git link.
• [x] Use GitLink! to open git link in browser.
• [x] Use GitLink blame to copy the /blame git link.
• [x] 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[bot]]

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).

:exclamation: 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 :warning:

Additional details and impacted files

```diff @@ 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 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.