`bundled.js` stopped being ES5 when the webpack build was replaced by tsdown
in #628. It parses at ecmaVersion 2015 but not 5:
var t=(t,e)=>()=>(e||(t((e={exports:{}}).exports,e),t=null),e.exports)
That is rolldown's `__commonJS` interop helper. `@rolldown/plugin-babel` only
transforms input modules, and rolldown appends the helper afterwards; terser
with `ecma: 5` avoids introducing newer syntax but does not transpile, so the
arrow functions reached the published file. `es5.js` has no CommonJS
dependencies and never gets the helper, which is why only `bundled.js` broke.
The effect is total rather than partial: in an ES5 engine the whole script is
a SyntaxError, so `bundled.js` — the bundle that exists specifically to serve
those engines, polyfills included — does not load at all there.
Lower the emitted chunk with a babel renderChunk pass that runs after bundling
and before terser, so rolldown's own helpers are covered too. Costs 2.8 kB
(+1.6%) on bundled.js; es5.js is unchanged at 34 kB.
The existing guard was a grep for backticks, which this syntax slips straight
past. Replace it with an acorn parse at ecmaVersion 5 over both legacy
bundles, plus a tokeniser check for real template literals (backticks inside
core-js string literals are fine, and 2.14.1 shipped three of them).
Verified by reverting the build fix: `bundled.js parses as ES5` fails, and
passes again once restored. Also confirmed bundled.js loads and parses the
live navigator.userAgent in a real browser.
The 'module' field is used by bundlers such as rollup and webpack to
determine the entrypoint for a package. Conventionally 'module' is used
for the ES6 entrypoint, and 'main' is used for the CommonJS entrypoint.
Adding a 'module' field allows importing bowser as an ES6 module
directly, rather than using a package-relative path to the ES6
entrypoint (e.g. `bowser/src/bowser`)
Closes#353
