Merging part of Dec 24, 2021 upstream commit

Improves syntax synchronization

Merging part of Dec 3, 2019 upstream commit

Adds support for spell checking

Fix my fork

The previous commit from upstream fundamentally broke my fork. While the
new method of matching tags is better, it treats beginning and end tags
as equivalent.

I have once again re-worked the handling of end tags to acheive the same
effect.

Merging part of Oct 26, 2019 upstream commit

Reworking of XML end tag handling.

This does break my fork.

Clean up credits

De-emphasize end tag

My preference is for the end tag to be less visible. This makes that
happen.

Half of the time, my XML files are actually XHTML, and I am closing
blocks with...

</div> <!-- class name -->

Highlighting these lines with differing colors is distracting, and it
would be too difficult to find a way to pattern match these types of
comments. Therefore my preference is to not highlight at all.

Updates

I don't know the history of these changes, but this is how I found the
XML vim syntax file when I began my fork.

Initial commit following rebuild