Update constants section to talk about inlining

[aslilac]

https://gleam.run/book/tour/constants.html

When a constant is referenced the value is inlined by the compiler

This is true for Erlang, but not for JavaScript, and it would probably make more sense to update this documentation than the change the JavaScript generation, because there are actually a lot of reasons why JavaScript constant inlining would be bad.

[lpil]

Yup yup, this is outdated documentation. Thank you!