diff-javascript code block generates invalid HTML

[marvinhagemeister]

Noticed on my blog that every diff block generates unclosed <span>-elements.

```diff-javascript
- foo();
```

I was able to trace the problem down to this particular line:

eleventy-plugin-syntaxhighlight/src/markdownSyntaxHighlightOptions.js

Line 28 in ac92151

let lines = html.split("\n").slice(0, -1); // The last line is empty.

It makes the assumption that the last entry in the array is always an empty string, but in my case that's not true. For me the last entry is "</span>". The slicing ignores that and thereby passes invalid HTML from here on.

Putting the slicing behind a check if the last line really is empty solves the problem for me.

let lines = html.split("\n");

// Trim last line if it is empty
if (lines[lines.length - 1] === "") {
  lines = lines.slice(0, -1);
}

I tried reproducing that with the test setup in this repository, but I was unable to do so. I can reproduce the error in a full eleventy setup though.

Git repo where the problem is reproducible: https://github.com/marvinhagemeister/eleventy-diff-syntax-highlight-bug

Steps to reproduce:

1. clone https://github.com/marvinhagemeister/eleventy-diff-syntax-highlight-bug and cd into it
2. npm install
3. npm run build
4. Generated file dist/foo/index.html contains invalid HTML

[zachleat]

Great reproduction and recommendation! This will ship with v5.0.0 (today)