correct BarSelect.value types to match option keys

[pklito]

BarSelectM and BarSelectOptions's attribute value stores the currently selected / default key in the options attribute, which is of type Record<string, T>.
This PR just changes the typing of the attributes in the BarSelectOptions class and the BarSelect class to reflect their usage.

currently:

let barItem1 = new BarSelect<number>('select_id', {
  options: {
      "a": 1,
      "b": 2
  },
 //linter error: "The expected type comes from property 'value' which is declared here on type 'BarSelectOptions<number>'"
      value: "a",   
  });

This previously gave a linter error but compiles and has no runtime errors, and conversely, value: 1 showed no linter error yet raised an error at runtime

Let me know if I'm wrong or I've made a mistake here, as I've only recently started learning plugins for blockbench,
Thanks!