crate-level attributes without a main function are not up-lifted in the playground like rustdoc does

[tesuji]

For example, https://doc.rust-lang.org/nightly/unstable-book/library-features/asm.html#inputs-and-outputs

I found the code that might deal with it:

mdBook/src/theme/book.js

Line 121 in a884c25

if (text.indexOf("#![feature") !== -1) {

[tesuji]

It happened because the hidden fn main inserted to the snippet,

[ehuss]

Ideally mdbook should emulate what rustdoc does here. I believe rustdoc lifts any inner attributes to the crate level, and it seems reasonable that mdbook should do the same.