Remove $ prompts when copying terminal commands

[JairajJangle]

QOL improvement for online doc interaction - prevent $ prompt char from being copied with the copy to clipboard button on code blocks.

This might not be the most elegant way to tackle this; however, the other solution was to introduce HTML to specifically exclude the $ char from the code block and thus from being selected at all. However, that would break the printability of the book.

For reference:

Earlier, if you pressed the copy button, it would copy the entire line like so:
$ curl --proto '=https' --tlsv1.2 https://sh.rustup.rs -sSf | sh

After the improvement, the copied text would be:
curl --proto '=https' --tlsv1.2 https://sh.rustup.rs -sSf | sh

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Copying of texts without $ is unaffected.

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Copying of texts having $ but aren't necessarily shell code is smartly exempted. So, if you copy this:

It would remain as is:

$ cargo run
   Compiling functions v0.1.0 (file:///projects/functions)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 1.21s
     Running `target/debug/functions`
The value of x is: 5

[JairajJangle]

Closing due to: #4108 (comment) :(

[JairajJangle]

Nvm, closing this for good after checking the long history of failures to tackle this particular QOL issue. + The mdBook(upstream as everyone says) is no longer accepting PRs on new issues as per the contribution guide:

Only issues that have an E-Help-wanted or Feature accepted label will likely receive reviews.