Bug#1023328: janus: Fail to build with rollup 3

[yadd at debian.org]

Package: janus
Severity: important
User: pkg-javascript-devel at lists.alioth.debian.org
Usertags: rollup-3

Sent with mass-bug

janus's build failed with rollup 3. This is often due to a bad
rollup.config.js which was accepted before (ES in a .js file).

To fix it, try to move rollup.config.js into rollup.config.mjs or
rewrite it into CommonJS.

rollup 3.x is available in experimental branch.