Depending on what you want to do, there are two options.
Format in Script Only
My experience with Javascript and date formatting has never been a favourable one, not like Excel or any of the Microsoft formatting options across it's platforms you typically have access to.
When it comes to working with Javascript dates, all I could find was cobbling the format together by using a string based approach.
You first need to turn the Excel date into a JS date and then do the work from there.
Try this ...
function main(workbook: ExcelScript.Workbook)
{
let selectedSheet = workbook.getActiveWorksheet();
let v = selectedSheet.getRange("A2").getValue() as number;
let javaScriptDate = new Date(Math.round((v - 25569) * 86400 * 1000));
const months = ["Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"];
let formattedDate = javaScriptDate.getFullYear() + '-' + months[javaScriptDate.getMonth()];
console.log(formattedDate)
}
Microsoft actually give you a small headstart with this documentation which I have applied to my answer ...
https://learn.microsoft.com/en-us/office/dev/scripts/resources/samples/excel-samples#dates
It's not ideal but (I believe) it's your best option.
To apply that to entire range/column, you'd need to use a loop which is far from ideal.
Apply Format to Entire Column
It's pretty simple ...
function main(workbook: ExcelScript.Workbook)
{
let selectedSheet = workbook.getActiveWorksheet();
selectedSheet.getRange("A:A").setNumberFormat("yyyy-MMM")
}
... you just need to know which column you want to format it on.