Bug 1825208 - [devtools] Fix pretty printing inline script on first line....

Bug 1825208 - [devtools] Fix pretty printing inline script on first line. r=devtools-reviewers,ochameau.

If a `<script>` is on the first line of the HTML file, we were miscalculating
its position in the HTML file because there is not previous line breaks.
A test case is added to fix this.

Differential Revision: https://phabricator.services.mozilla.com/D173929