Typing / after a < when we know the tag isn't closed should close the open tag.

[charlespwd]

Is your feature request related to a problem? Please describe.
This is a followup of #501 (comment)

It seems like the HTML Language Server also does html/autoInsert when typing / after a <.

Describe the solution you'd like

• If the user types / after a < and we know that it's safe to close a tag that currently is opened without child, then we should complete it.
• We shouldn't close tags that are already opened, don't want duplicate close tags (no <div></div></div> pls)

We might be smart and do a "smart edit of the file", pass it through the parser and verify for a LiquidHTMLASTParsing error like we do for the > case.