jsgen: fix slightly incorrect JS (breaks esbuild) (fix #23711)

[gchumillas]

Fixes #23711

How to test in Linux (macos):

# install esbuild (if not already installed)
npm -g install esbuild

# compile and minimize the code
cd examples
v -skip-unused -backend js_browser -prod ./js_hello_world.v -o hello.js
esbuild --minify --platform=browser --bundle hello.js --outfile=hello.min.js

# run it
node hello.min.js

Output:

Hello from V.js (0)
Hello from V.js (1)
Hello from V.js (2)

[huly-for-github]

Connected to Huly®: V_0.6-23033

[spytheman]

-skip-unused should be on by default now.

[spytheman]

I've just checked, and it works on my machine, with latest esbuild as well, while failing on master: