message: fix(runtime): compat with noUncheckedIndexedAccess
  for projects using the new noUncheckedIndexedAccess typescript flag, `classes.root` gets a type of `string | undefined`, forcing the user to use `!` in many cases.
  this change relaxes the type-check of the first parameter, at least until we have .d.ts generation with watch mode.
diff: @@ -22,7 +22,7 @@ export interface StylableExports {
  }
  export type STFunction = (
  - context: string,
  + context: string | undefined,
  stateOrClass?: string | StateMap | undefined,
  ...classes: Array<string | undefined>
  ) => string;
  