[F] setExtension
function setExtension(   program,   target,   key,   value): void;Set extension on the given type(Same as calling @extension decorator)
Parameters
| Parameter | Type | Description | 
|---|---|---|
| program | Program | TypeSpec program | 
| target | Type | Type | 
| key | string | Extension key | 
| value | unknown | Extension value | 
Returns
void