feat: migrate bundler from microbundle to rolldown (#588)

* feat: replace microbundle with rolldown

- Remove microbundle, add rolldown as dev dependency
- Add rolldown.config.ts with ESM + CJS output for all entry points
- Update build scripts to use rolldown
- Update exports/browser/module fields for new output paths
- Remove source and unpkg fields (microbundle-specific / UMD dropped)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* build: rebuild dist with rolldown

- Single .mjs instead of modern.mjs + module.mjs
- Remove UMD output files
- Output is no longer minified (ESNext syntax)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* docs: update README and HISTORY for v4.0.0

- Remove browser/UMD usage section from README
- Add v4.0.0 changelog entry to HISTORY

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: sasaplus1

HISTORY.md

@@ -1,3 +1,11 @@
+# 4.0.0 / 2026-02-14
+
+- migrate bundler from microbundle to rolldown
+- drop UMD output
+- drop `unpkg` field from package.json
+- unify ESM output (`modern.mjs` + `module.mjs` → single `.mjs`)
+- output is no longer minified (ESNext syntax, no ES5 downlevel)
+
 # 3.0.2 / 2026-01-25
 
 - fix types

README.md

@@ -18,12 +18,6 @@ $ npm install ltsv
 import * as ltsv from 'ltsv';
 ```
 
-### browser
-
-```html
-<script src="ltsv.min.js"></script>
-```
-
 ### Example
 
 ```js