Doc test generator - Githubissues

A quick little tool that visits all .mdx files, parses them using pulldown-cmark, and writes all codeblocks annotated with something like:

```rust template="core"

into a separate test file, that applies the core template.

There is nothing fancy about this, just some globbing, aggregating, and file creation.
Also supports Go blocks, but I'm not sure how to properly set up a working test environment for this, I'm just a poor little Rust dev.

This should either be a Git hook or we should have a CI step that ensures no new files have been created (probably both).

CosmWasm / docs

Doc test generator #15