gh-136938: Do not use var in js, use let and const

[sobolevn]

cc @hoodmane

• Issue: Don't use var in js code #136938

[sobolevn]

!buildbot emscripten

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @sobolevn for commit 69e0d6c 🤖

Results will be shown at:

https://buildbot.python.org/all/#/grid?branch=refs%2Fpull%2F136939%2Fmerge

The command will test the builders whose names match following regular expression: emscripten

The builders matched are:

• WASM Emscripten PR

[hoodmane]

The change to Python/sysmodule.c looks good to me. As far as Python/emscripten_syscalls.c, I think I'd prefer to keep the code as close as possible to the reference Emscripten code which uses var.

[freakboy3742]

Agree with @hoodmane - it makes sense to clean up the "Python internal" usage; but when there's code being synchronised with upstream, we should (to the extent possible) keep the var/let/const usage consistent.