Restore TypeScript support for Observable styles

[appsforartists]

Corresponding issue (if exists):

What would you like to add/fix?

const jss = createJSS(createDefaultJSSPreset());
jss.createStyleSheet(
  {
    myClassName: someObservable$,
  },
  {
    link: true,
  },
).attach();

should work in TypeScript, and used to work with @types/jss. JSS now supplies its own types, which don't include Observable support. This PR adds them back.

As @worudso alluded to in #1035, a better solution would only include the Observable type if users had installed the Observable plugin. Otherwise, users could be frustrated if the typecheck passes for createStyleSheet(someObservables) but it fails at runtime.

However, that solution would require bigger and more thoughtful changes to the typings. This PR is a quick fix that addresses the regression from @types/jss, while leaving the work of a refactor to a future PR.

Todo

• Add tests if possible
• Add changelog if users should know about the change
• Add documentation

[kof]

lgtm, thanks!

[kof]

released https://github.com/cssinjs/jss/releases/tag/v10.5.0