[Feature] Add support for sheet.getSelections()

[F-loat]

Initial checklist

• Is this really a problem?
• I have searched the Github Issues for similar issues, but did not find anything.

Problem

Currently I can only get the last selected range through the api.

const workbook = univerAPI.getActiveWorkbook();
const sheet = workbook?.getActiveSheet();
const selection = sheet?.getSelection();
const range = selections?.getActiveRange();

But I need to get all the selected ranges.

const workbook = univerAPI.getActiveWorkbook();
const sheet = workbook?.getActiveSheet();
const selections = sheet?.getSelection()?._selections;
const ranges = selections.map(({ range }) => range);

The above code works, but it uses private properties.