feat: switch from tsup to tsdown

[JoshuaKGoldberg]

PR Checklist

• Addresses an existing open issue: fixes 🚀 Feature: Switch from tsup to tsdown #2223
• That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

Switches over, and includes intake/transition logic to remove the bundle property from old tsup.config.ts files.

Blocked from passing lint:spelling on streetsidesoftware/cspell-dicts#4518.

🎁

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.80%. Comparing base (5ed6338) to head (adad593).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2224   +/-   ##
=======================================
  Coverage   99.80%   99.80%           
=======================================
  Files         136      136           
  Lines        6061     6072   +11     
  Branches      505      507    +2     
=======================================
+ Hits         6049     6060   +11     
  Misses         12       12           

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