feat(api): add 'link' api

[linrongbin16]

Resolve #213 .

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 72.15190% with 22 lines in your changes are missing coverage. Please review.

Project coverage is 76.42%. Comparing base (38b508e) to head (83b2ad7).

Files	Patch %	Lines
lua/gitlinker.lua	69.44%	22 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #215      +/-   ##
==========================================
+ Coverage   71.38%   76.42%   +5.03%     
==========================================
  Files           9        9              
  Lines         734      772      +38     
==========================================
+ Hits          524      590      +66     
+ Misses        210      182      -28     

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