fix: deprecate underscore in favor of underline

[sdanialraza]

Please describe the changes this PR makes and why it should be merged:
This addresses the issue where the formatter is named underscore and its escaper is named escapeUnderline as pointed out in Discord server. I went with underline because that's what Discord refers it as in Markdown Text 101.

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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[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (651ffc2) 58.34% compared to head (2c2ff6a) 58.31%.
Report is 4 commits behind head on main.

Files	Patch %	Lines
packages/formatters/src/formatters.ts	81.81%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10054      +/-   ##
==========================================
- Coverage   58.34%   58.31%   -0.03%     
==========================================
  Files         241      241              
  Lines       17158    17154       -4     
  Branches     1242     1240       -2     
==========================================
- Hits        10010    10004       -6     
- Misses       7103     7105       +2     
  Partials       45       45              

Flag	Coverage Δ
builders	95.53% <ø> (ø)
formatters	99.27% <81.81%> (-0.21%)	⬇️
next	∅ <ø> (∅)

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