Add Vim swap file

[GangWang01]

Reasons for making this change: As a common template .gitignore maintained by https://github.com/dotnet/templating, the customer requested to add Vim swap files to it. To keep both sides aligned, request to add Vim swap files to this template.

Links to documentation supporting these rule changes:

https://github.com/dotnet/templating/issues/5048

[kkirsche]

Is there a reason that this cannot leverage https://github.com/github/gitignore/blob/main/Global/Vim.gitignore?

There are a few tools like gitignorer and whatnot to help with concatenating multiple gitignore files together