Exclude npm pack artefacts in gitignore (DefinitelyTyped#44454)

This prevents accidental inclusion of common artefact:

see:
https://github.com/github/gitignore/blob/master/Node.gitignore#L65
DefinitelyTyped#44439 (comment)

Thanks!

Co-authored-by: PiotrB <[email protected]>

Author: peterblazejewicz

.gitignore

@@ -57,3 +57,6 @@ yarn.lock
 shrinkwrap.yaml
 pnpm-lock.yaml
 pnpm-debug.log
+
+# Output of 'npm pack'
+*.tgz