feat(link): add external prop to SLink (#293)

[NozomuIkuta]

Resolves #293

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[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.02 🎉

Comparison is base (864c9c5) 82.89% compared to head (0dd01fd) 82.91%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #294      +/-   ##
==========================================
+ Coverage   82.89%   82.91%   +0.02%     
==========================================
  Files         120      120              
  Lines       10106    10119      +13     
  Branches      420      423       +3     
==========================================
+ Hits         8377     8390      +13     
  Misses       1729     1729              

Impacted Files	Coverage Δ
lib/components/SLink.vue	100.00% <100.00%> (ø)

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[kiaking]

I have:

• Fix :external default value issue (always false).
• Adjust story.
• Add test.
• Add docs.