Shift HtmlElement location by 1

The HtmlElement location will now refer to the start of tagname instead of the < character:

<div>
 ^~~

instead of

<div>
^~~~

The reasoning is that when marking the region a reported error refers to it looks weird when the < is included but not >.

Also consider attributes and multiline elements:

<div id="foo" class="bar">
 ^~~

<div id="foo"
 ^~~
     class="bar">

Checklist

• Changelog updated
• Documentation updated
• Change covered by a testcase
• Commit history cleaned (no WIP, fixups, etc)