Implement inline line numbering

Right now, if you want line numbers, the best way is to use `include code` to embed code from GitHub. But it would be nice to have a robust option for doing line numbering for inline code snippets as well. (There is `{% highlight linenos %}` but it [apparently has problems](https://hydejack.com/docs/writing/#adding-code-blocks).) See [this blog post](https://www.bytedude.com/jekyll-syntax-highlighting-and-line-numbers/) for a better way forward.

Start by reading the existing `{% highlight linenos %}` approach and the linked Jekyll syntax-highlighting blog post. Inspect how inline code snippets are rendered and define a robust line-numbering option that works for them; done means inline snippets display line numbers without relying on GitHub includes or the documented highlight workaround.

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