Extract code blocks from .rst files and create a global Rust test file (generated.rs) to check them all together using
rustc --test --edition=2021 exts/rust-code-runner/generated.rs under the hood.

Features:

• marker comments to hide specific code from being rendered in html:

      .. code-block:: rust

        // HIDDEN START
        use std::fs;
        use std::io::{self, Read};
        // HIDDEN END 

anything between // HIDDEN START and // HIDDEN END will not be rendered but will still be included for compilation in the generated test file.

• This extension is not tied to the other 'needs' extension so it can be used in other projects that require similar features.
• A set of functions that parse rustc --error-format=json output and extract minimal error information.

Tasks:

• Write unit tests -- this mostly uses regex(scary) for extracting code blocks.
• Comments
• Refactor
• Feature-gate running code, for CI integration.

How to test:

Run: ./make.py -c --offline it will notify you in case there is any issue in any code block,
also check exts/rust-code-runner/generated.rs after doing so.

(Not rebased yet)
Resolves #80