Allow build commands to run under Windows

[scrhartley]

Description

Allow running the build commands on Windows.

On Windows, even under WSL, it can't find sed from within the package.json scripts.
In addition, Windows doesn't have glob expansion, so instead it needs to get brotli-cli to do it.

Due to line ending differences, the brotli versions of the non-minified JS files don't match exactly when generated on Windows. (Minification seems to auto-convert Windows line endings to Linux ones, but the brotlification doesn't.) I tried playing about with Git auto eol conversation config, but I didn't manage to win that battle.

Testing

I wiped out the contents of the dist folder, ran npm run build and then looked at the git diffs to see what changed.

Checklist

• I have read the contribution guidelines
• I have targeted this PR against the correct branch (master for website changes, dev for
  source changes)
• This is either a bugfix, a documentation update, or a new feature that has been explicitly
  approved via an issue
• I ran the test suite locally (npm run test) and verified that it succeeded