Use MfmUrl AST in MfmLink AST

[shibaobun]

What

• Use MfmUrl AST in MfmLink AST
• Add test cases for reproducting bracket urls within links

Why

• This fixes issues with brackets disappearing
• Closes #83

Additional info (optional)

This is my first time using typescript, so please let me know if there's anything that can be changed for best practice! Thank you!

[codecov[bot]]

Codecov Report

Merging #126 (88daccc) into develop (ae92c2a) will increase coverage by 0.00%. The diff coverage is 100.00%.

:exclamation: Current head 88daccc differs from pull request most recent head e7f70c6. Consider uploading reports for the commit e7f70c6 to get more accurate results

@@           Coverage Diff            @@
##           develop     #126   +/-   ##
========================================
  Coverage    97.72%   97.73%           
========================================
  Files            8        8           
  Lines         1541     1544    +3     
  Branches       317      233   -84     
========================================
+ Hits          1506     1509    +3     
- Misses          33       34    +1     
+ Partials         2        1    -1     

Impacted Files	Coverage Δ
src/internal/parser.ts	99.33% <100.00%> (ø)
src/internal/util.ts	97.63% <100.00%> (ø)
src/node.ts	100.00% <100.00%> (ø)
src/api.ts	100.00% <0.00%> (ø)
src/internal/index.ts	100.00% <0.00%> (ø)

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[shibaobun]

@syuilo i've rebased this PR with your changes, please let me know if there's anything you'd like changed!