`security` -> `severity`

I realised I was reading it wrong after searching `severity` but couldn't find one, then realised it's called `security`. But I think we should definitely fix it.

It should be a backward compatible one by just adding the new option and assign old option to the new one. Then search & replace the docs.

Locate the TypeScript configuration handling the existing `security` option and search the documentation for references to that name. Add the backward-compatible `severity` option, preserve the old option's behavior, and update the documentation so the new name is used consistently.

Written by the indexing model from the issue text.