v11: bump browser UMD/min compile target to ES6 and remove downlevelIteration

[harttle]

Context

Browser UMD/min bundles currently compile with Rollup target: es5, which requires downlevelIteration: true in the Rollup TypeScript override so generator-based code type-checks and emits correctly.

This was reintroduced when removing deprecated suppressImplicitAnyIndexErrors from tsconfig.json (see rollup tsconfig() helper).

v11 task

• Change browser UMD/min Rollup targets from es5 to es6
• Remove the es5-only downlevelIteration: true override from rollup.config.mjs
• Confirm browser bundle output and supported environments

Notes

• Root tsconfig.json already uses "target": "es6"
• ESM bundles already compile with es6
• Dropping ES5 browser output is a breaking change for very old browsers; document in v11 release notes

[harttle]

Implemented on the
ext\ branch (v11) in #933 — downlevelIteration removed from Rollup tsconfig override (ES6 UMD/min target work).