Add expression support for #-name shorthand

[villor]

bevyengine#20158

The #-syntax currently allows specifying names using Idents, which is nice.

This PR extends it to also support arbitrary expressions enclosed in braces:

let n = 5;
bsn! {
     #{format!("Tab_{n}")} Node
}

[cart]

I like it! I had inklings of a plan to do the same thing :)

[villor]

If it turns out string interpolation using #{format!("...")} is a very common occurence
A future idea is to also have #"..." automatically expand like #{format!("")}:

let n = 5;
bsn! {
     #"Tab_{n}" Node
}