Migrate from rolldown to tsdown

[nstepien]

• rolldown dropped support for CSS, unclear when it'll be restored, if ever
• vite's library mode does not support outputting declaration files
• tsdown, built on top of rolldown, is the ideal lib bundling tool for rdg

https://tsdown.dev/
https://github.com/rolldown/tsdown

before

<DIR>/styles.css      asset │ size:   9.78 kB
<DIR>/styles.css.map  asset │ size:  12.75 kB
<DIR>/index.js.map    asset │ size: 216.46 kB
<DIR>/index.d.ts      chunk │ size:  23.08 kB
<DIR>/index.js        chunk │ size:  99.18 kB

✔ rolldown v1.0.0-rc.5 Finished in 481.46 ms

after

ℹ tsdown v0.21.7 powered by rolldown v1.0.0-rc.12
ℹ config file: D:\repos\react-data-grid\.claude\worktrees\vitelib\tsdown.config.ts 
ℹ entry: src\index.ts
ℹ tsconfig: tsconfig.json
ℹ Build start
ℹ Cleaning 5 files
ℹ lib\index.js       98.39 kB │ gzip: 21.95 kB
ℹ lib\index.js.map  217.24 kB │ gzip: 53.86 kB
ℹ lib\styles.css      8.22 kB │ gzip:  2.12 kB
ℹ lib\index.d.ts     23.10 kB │ gzip:  5.14 kB
ℹ 4 files, total: 346.95 kB
✔ Build complete in 187ms

build output changes

We've lost the sourcemaps for CSS files, not a major concern.

In the JS, besides empty lines being removed, some const values are inlined:

In the declaration file, the name of namespace imports changed, that's it:

The CSS file is processed by lightningcss, so it has more changes, but nothing major, some highlights: