add support to literal type

[AllyW]

Refer to autorest literal type parsing: https://github.com/Azure/typespec-azure/blob/main/packages/typespec-autorest/src/openapi.ts#L2358-L2373
test property:

  @doc("The datasource type")
  dataSourceType: "AzureVM";

  @doc("The datasource Int")
  dataSourceInt: 12;

  @doc("The datasource Float")
  dataSourceFloat: 12.5;

to:

"props": [
                            {
                              "type": "string",
                              "enum": {
                                "items": [
                                  {
                                    "value": "AzureVM"
                                  }
                                ]
                              },
                              "name": "dataSourceType",
                              "description": "The datasource type",
                              "required": true
                            },
                            {
                              "type": "integer",
                              "enum": {
                                "items": [
                                  {
                                    "value": 12
                                  }
                                ]
                              },
                              "name": "dataSourceInt",
                              "description": "The datasource Int",
                              "required": true
                            },
                            {
                              "type": "float",
                              "enum": {
                                "items": [
                                  {
                                    "value": 12.5
                                  }
                                ]
                              },
                              "name": "dataSourceFloat",
                              "description": "The datasource Float",
                              "required": true
                            }
]