feat: add subtext formatter

[sdanialraza]

Please describe the changes this PR makes and why it should be merged:
Adds a formatter for the new subtext markdown feature which can be found here under "Organizational Text Formatting"

Status and versioning classification:

• Code changes have been tested against the Discord API, or there are no code changes
• I know how to update typings and have done so, or typings don't need updating
• This PR changes the library's interface (methods or parameters added)

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2 Skipped Deployments

Name	Status	Preview	Comments	Updated (UTC)
discord-js	⬜️ Ignored (Inspect)	Visit Preview		Jul 24, 2024 10:17am
discord-js-guide	⬜️ Ignored (Inspect)	Visit Preview		Jul 24, 2024 10:17am

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 50.55%. Comparing base (efa16a6) to head (bc15654).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10400      +/-   ##
==========================================
+ Coverage   50.52%   50.55%   +0.02%     
==========================================
  Files         228      228              
  Lines       20705    20715      +10     
  Branches     1252     1253       +1     
==========================================
+ Hits        10462    10472      +10     
  Misses      10199    10199              
  Partials       44       44              

Flag	Coverage Δ
builders	95.78% <ø> (ø)
formatters	99.34% <100.00%> (+<0.01%)	⬆️

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