feat(build): adopt tsdown for builds

[matthewgallo]

Closes #8024

📢 This branches off #7956 which should be reviewed and merged before this PR

This replaces our rollup build setup with tsdown for both react and web components. There was a noticeable improvement in the react build times (~25 seconds down to ~3.5 seconds). The web component package requires some sass transformation in order to map the sass files into lit templates which uses sass.render which is very slow and also deprecated. If we could find a faster alternative to sass.render the web components package would likely see stronger improvements.

What did you change?

packages/ibm-products/tsdown.config.ts
packages/ibm-products/package.json
packages/ibm-products-web-components/tsdown.config.ts
packages/ibm-products-web-components/package.json
yarn.lock

*Deleted previous rollup build scripts for both react and web component packages

How did you test and verify your work?

Manually ran builds for both react and web component packages

PR Checklist

As the author of this PR, before marking ready for review, confirm you:

• Reviewed every line of the diff
• Updated documentation and storybook examples
• Wrote passing tests that cover this change
• Addressed any impact on accessibility (a11y)
• Tested for cross-browser consistency
• Validated that this code is ready for review and status checks should pass

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[codecov]

Codecov Report

❌ Patch coverage is 62.50000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.21%. Comparing base (1c4fa62) to head (5eef4e6).
⚠️ Report is 36 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8065      +/-   ##
==========================================
- Coverage   79.38%   79.21%   -0.17%     
==========================================
  Files         474      475       +1     
  Lines       20340    20517     +177     
  Branches     5006     5024      +18     
==========================================
+ Hits        16146    16253     +107     
- Misses       4194     4264      +70     

Components	Coverage Δ
ibm-products	80.48% <71.79%> (-0.10%)	⬇️
ibm-products-web-components	76.44% <68.30%> (-0.28%)	⬇️

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