Function to initialise google analytics html script

[rmbielby]

Adding a script to create a basic google-analyics.html file with a provided GA4 key.

The function initialise_analytics() creates the script and I've added some validation to check he provided key at least looks like a genuine GA4 key. I've also added some tests to make sure that validation is working.

[rmbielby]

Few minor things in comments, some lintr things to either dismiss or address, plus a couple of notes returned when running devtools::check() locally, I've commented on some in line, but there's one more too:

Not 100% sure on this, but worth a quick check if we're doing something wrong?

Think I've covered everything above if you want to have another look through.

[cjrace]

Analytics initialisation function