Update Stata.gitignore to ignore new auto backup files

Stata 18 introduced an auto-backup feature which creates a temporary file while a code file is open it its IDE.

These auto-backup files are analogous to the [Microsoft Office backup files that are gitignored](https://github.com/github/gitignore/blob/main/Global/MicrosoftOffice.gitignore).

[Ref](https://www.stata.com/manuals/gsm13.pdf):
> The Do-file Editor now creates a backup file whenever it opens a document or creates a new one.
> When an existing document is opened, Stata creates a backup file of a document that is saved to disk
> in the same directory using the existing document’s filename prefixed with ~ and with the extension
> .stswp. When you edit a new and unsaved document, it saves the backup file to the temp directory.
> When a document is closed, the backup file is deleted. However, if Stata does not exit cleanly because
> of a power outage or a computer crash, the backup file is left behind.