add better support for `"source.fixAll"`

[Techatrix]

If you don't add "source.fixAll" to "editor.codeActionsOnSave" autofix won't work in VS-Code (workaround in #1094). It would be helpful if vscode-zig did this for you, but I'm not sure how to achieve this without also enabling "source.fixAll" on other languages. I think you could consider this an oversight by VS-Code that there is not way to enable this on a per language level which is probably why as an example eslint is using "source.fixAll.eslint". So we either fix VS-Code or add "source.fixAll.zls"?