build(client): generate a UMD bundle

[lpinca]

First commit

build(client): generate a UMD bundle
    
A UMD bundle can be used as a regular CommonJS module by bundlers and
included in other bundles.

Second commit

build(client): drop the .mjs extension
    
The .mjs extension is misleading as the generated bundle is not an
ECMAScript module and prevents bundlers from working properly.