Fix codeblock in codeblock

[dalance]

Codeblock which contains ``` will be like below:

````markdown
```rust
let a = 0;
```
````

But the output of mdbook-i18n-helpers seems to be broken like below:

```markdown
```rust
let a = 0;
```
```

This is because codeblock token count is fixed as 3 in reconstruct_markdown.
This PR adds to check the appropriate codeblock token count.

[mgeisler]

Thanks @dalance for finding and fixing this! I believe the correct place to fix this would actually be in pulldown-cmark-to-cmark. I went looking there and Byron/pulldown-cmark-to-cmark#20 looks related.

I'm happy to fix this here for now, but we should add a TODO comment with a reference to pulldown-cmark-to-cmark to see if we can fix it for everybody.

[mgeisler]

Suggested change

	/// Check appropriate codeblock token count
	fn check_code_block_token_count(group: &[(usize, Event)]) -> usize {
	/// Check appropriate codeblock token count
	///
	/// See https://github.com/Byron/pulldown-cmark-to-cmark/issues/20
	/// for a related upstream issue.
	fn check_code_block_token_count(group: &[(usize, Event)]) -> usize {

[dalance]

@mgeisler Thank you for merging this PR.
Could you release 0.3.1 including this?
(Or is there any release plan?)

This is because the problem is a blocking issue of translation on https://github.com/rust-lang/rust-by-example.

[mgeisler]

@mgeisler Thank you for merging this PR. Could you release 0.3.1 including this? (Or is there any release plan?)

This is because the problem is a blocking issue of translation on https://github.com/rust-lang/rust-by-example.

Thanks for reminding me of this! Let me create a release now!

[mgeisler]

I've made a 0.3.1 release! 🎉