This change fixes a few issues with `elk-api.d.ts.`:

1. Identifiers in ELK JSON can be a string or an integer [according to the documentation](https://eclipse.dev/elk/documentation/tooldevelopers/graphdatastructure/jsonformat.html#nodes-ports-labels-edges-and-edge-sections).

> The id can be a string or an integer.

Because of this, a new `type ElkIdentifier` was added and used where necessary.

2. Missing semicolons in the definitions of the interfaces. Although there is no up-to-date specification for typescript, the majority of examples shown in the official documentation have semicolons. There is also inconsistent use of semicolons in this file so this fixes that as well.

Update LayoutOptions so that values such as:

```
'layered.spacing.nodeNodeBetweenLayers': 40
```

is supported.