Enhancement: Retry frontend highlighter init later

[pattonwebz]

This PR adds a simple retry later to init the Frontend Highlighter. This should solve the issue where Cloudflare Rocket Loader defer prevents the highlighter from initialising.

It will try init first on DOMContentLoaded and then later at load. It adds a variable as a flag to track if the init code has already run to prevent it duplicating on load if it already ran when initially intended.

Checklist

• PR is linked to the main issue in the repo
• Tests are added that cover changes

[SteveJonesDev]

A minor improvement could be to change initHasRun to highlighterInitialized so it's more descriptive. Either way is fine, I'll let you decide.