Add tabindex and focus states to code blocks

[richardTowers]

This is required for accessibility reasons. Because some code blocks have scrollbars they need to be focusable using a keyboard.

I've copied the styles from the design system's documentation (see the code blocks in the HTML / Nunjucks tabs here: https://design-system.service.gov.uk/components/button/)

The implementation is simple when syntax highlighting is not enabled, because we can simply implement block_code to surround the code with the markup we want.

When syntax highlighting is enabled it's a bit more complicated. Middleman already includes its own implementation of block_code, which renders the source code highlighted with spans. This method doesn't look like it would be easy to customise, so instead I've resorted to post-processing the HTML (i.e. replacing the attributes on the <pre> tag using a regex).

Testing this was a bit tricky, because you need to monkey patch the class to include the block_code method. I had to clone the class and patch the clone to do avoid affecting other tests.

Example from a locally running version of the paas tech docs:

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⚠️ Don't forget to update the gem version in the CHANGELOG before merging! When you're ready to release bump version file and generate a tag. ⚠️

[AlanGabbianelli]

Looks good 👍
Just a note on the test organisation and semantic. It would be easier to follow with a couple of minor changes:

• only one describe block to describe what you're testing
• two context blocks to specify the conditions of the tests

It would look something like this:

  describe "#render a code block" do
  	context "without syntax highlighting" do
      # setup for conditions without syntax highlighting

      it "sets tab index to 0"

      it "renders the code without syntax highlighting"

    context "with syntax highlighting" do
      # setup for conditions with syntax highlighting

      it "sets tab index to 0"

      it "renders the code with syntax highlighting"
    end
  end

[richardTowers]

Thanks! Addressed in a0c9bfd