Add guard against reloading a file that's not a variants file

[jophippe]

This change addresses item #3727

The following changes are proposed:

• Add an explicit guard when reloading the variants file to make sure that the file we're loading is actually a variants file, so we don't try to treat any other file as a variants file and throw a parsing exception.

Other Notes/Information

Ideally, we would figure out why the reload variants functionality would be given a path other than a variants file, since our file watcher should only be watching for variants files, but I haven't figured that out yet

[jophippe]

@gcampbell-msft Okay, I made a 1.19 section in the changelog and moved both my bugfixes into it. Does anything else need to be moved into the 1.19 section?

[gcampbell-msft]

@gcampbell-msft Okay, I made a 1.19 section in the changelog and moved both my bugfixes into it. Does anything else need to be moved into the 1.19 section?

I think that's it