Add async flag to methods and functions

While subsequent tooling could infer this from the return type, @justinfagnani rightly pointed out that there are some semantic differences, notably for error types, and perhaps for early returns too.

API tables would benefit from this flag as well:


```json
{
"name": "mutate",
"async": true,
"summary": "This resolves a single mutation according to the options specified and returns a Promise which is either resolved with the resulting data or rejected with an error.",
"parameters": [{
"name": "params",
"optional": true,
"type": "Partial, Variables>>",
"summary": "All named arguments to mutate default to the element's corresponding instance property. So you can call `element.mutate()` without arguments and it will call using `element.mutation`, `element.variables`, etc. You can likewise override instance properties per-call by passing them in, e.g.\n\n```ts\nawait element.mutate({\n fetchPolicy: 'network-only'\n variables: {\n ...element.variables,\n name: 'overridden',\n },\n});\n```\n\n| Property | Description | Type |\n| -------- | ----------- | ---- |\n| `awaitRefetchQueries` | See [awaitRefetchQueries](#awaitrefetchqueries) | `ApolloMutation#awaitRefetchQueries` |\n| `context` | See [context](../element/#context) | `Record` |\n| `errorPolicy` | See [errorPolicy](../element/#errorpolicy) | `ErrorPolicy` |\n| `fetchPolicy` | See [fetchPolicy](#errorpolicy) | `FetchPolicy` |\n| `mutation` | See [mutation](#mutation) | `ApolloMutation#mutation`\n| `optimisticResponse` | See [optimisticResponse](#optimisticresponse) | `ApolloMutation#optimisticResponse`\n| `refetchQueries` | See [refetchQueries](#refetchqueries) | `ApolloMutation#refetchQueries`\n| `update` | See [updater](#updater) | `ApolloMutation#updater`\n| `variables` | See [variables](#variables) | `ApolloMutation#variables`\n"
}],
"return": {
"type": "Promise>>",
"summary": "A `FetchResult` object containing the result of the mutation and some metadata\n\n| Property | Description | Type |\n| -------- | ----------- | ---- |\n| `data` | The result of a successful execution of the mutation | `Data` |\n| `errors` | included when any errors occurred as a non-empty array | `readonly GraphQLError[]` |\n| `extensions` | Reserved for adding non-standard properties | `ApolloMutation#awaitRefetchQueries` |\n| `context` | See [context](../element/#context) | `Record` |\n"
}
}
```