Automate posting a diff of changes to the dist folder when creating a new release

[36degrees]

What

Using GitHub Actions, automatically post a prettified git diff of the dist folder on pull requests that update the dist folder.

Why

We've got in the habit of doing this as part of our release process, as it allows us to review the (minfied) changes to the dist folder, checking that they're what we expect.

For example, see the diff for the dist folder for v3.10.0.

However, this requires us to remember that we need to do it, and is a fairly manual process.

Who needs to know about this

Developers

Further detail

The current process uses the approach outlined here to prettify the minified JS / CSS before diffing it:
https://scripter.co/git-diff-minified-js-and-css/

1. Install js-beautify

2. Define two new diff drivers in the user's .gitconfig:

   [diff "minjs"]
       textconv = js-beautify
       cachetextconv = true
   [diff "mincss"]
       textconv = js-beautify --css
       cachetextconv = true
   

3. Configure git to use the diff driver for minified CSS and JS by adding to the .gitattributes file in the repo:

   *.min.js diff=minjs
   *.min.css diff=mincss
   

4. Run git diff against the master branch for the dist folder (using -M to detect renames with a similarity of 10% seems to work reliably so that git doesn't see the rename as a deletion and an addition)

   git diff -M1 master dist
   

5. Paste the output on the PR, using diff for syntax highlighting.

We should try and automate this, whilst ensuring that git diff continues to work for users without having to install or configure anything. For example, we should check whether committing the .gitattributes file to the repo would cause issues for users without a minjs or mincss diff driver.

Done when

• Prettified diffs are automatically posted on PRs that update files in diff